If you have any problems with your CPU, first you have to identify your make and model. If your computer can start windows right click on My Computer, click Properties and you will see listed your CPU make, model and speed.
If your CPU is damaged, Windows will crash, freez, or may not even start booting. The problem is that those symptoms appear also when your motherboard is damaged, so it's pretty hard to determine which one is to blame.
If you have recently installed a new processor or a heatsink, you may have not seated the processor correctly. If the processor sits at even a slight angle, the heatsink won’t sit flush against the CPU’s protective shield. Even small gaps between the heatsink and the processor will reduce your heatsink’s ability to cool your CPU. Remove the heatsink and then inspect the CPU and CPU socket.
Motherboard sockets have a lever or similar mechanism that locks the processor firmly into the socket. Lift this lever and then press the edges of the processor gently to make sure it is flush with the socket. Next, snap the lever back into place. Now remove the old thermal paste from the heatsink. You can use alcohol and a piece of cloth. Apply a thin and uniform layer of thermal paste over the heatsink.
Put the heatsink back to it's place according to it's instructions. Make sure that the heatsink power cable is connected before you start the computer. If your problem is not solve after properly placing the CPU in the socket and replacing the thermal paste, your CPU is not the problem.
Processors troubleshooting
Hard drive troubleshooting
When you suspect your hard drive is having problems, before you do anything else, check inside your computer to see if both power and data cables are well attached to their slots. Back up your personal data to another hard drive or removable disk.
Try updating your driver. If you have combined hard drives in a RAID, make sure you download and install a RAID driver.
Open My Computer and right click your hard drive. Choose Properties-Tools tab-Check Now (Error Checking). Confirm both checkboxes to perform a deeper scan of your drive, including every bit of it's disk surface. To scan your operating system drive may require a reboot.
If the scan turns up errors, restart your PC and enter BIOS. Enable SMART (Self-Monitoring Analysis and Reporting Technology) monitoring, if your hard drive supports it. Save your changes and exit.
Next use a different computer to download SeaTools from www.seagate.com and install it on a bootable CD-R or USB stick. Boot your damaged PC with the bootable CD-R. Use a PS/2 mouse to avoid problems about navigation in SeaTools. If you dont have one, use "tab", "spacebar", arrow keys and combination of ALT+underlined letter keys (ALT-N for next). After the utility loads, run a full scan of the suspect hard drive. Yellow and Red indicate problems.
If your hard drives has bad sectors, you have lost the files stored on them. SeaTools can overwrite those sectors, meaning that it will erase the data stored on them but also map out the defective areas so that they're never used again. Most drives develop a few bad sectors over time, but if your drive accumulates more bad sector every time you perform a scan, it's probably failing. If so, replace the drive with a new one. Western Digital - Black Caviar or Seagate Barracuda would be my first 2 choices.
If your problem is not about bad sectors, try the next few tips.
Run Disk Cleanup, from System tools, to free up storage space. Make sure that you have at least 1Gb of free space on your hard drive, or Windows will not run properly.
After this, try defragmenting the drive. Also in System Tools, choose Disk Defragmenter. Watch out, defragmenting your operating system drive can cause, rarely, problems such as file system errors.
Graphics card troubleshooting
P: System crash after running a video game or application that needs serios graphic resources.
A: Check your video card temperature. Some manufacturers have software for monitoring CPU or video card temperature. Check with your graphics card documentation for the highest temperature supported. Start your PC with the case open and check if your case fans and video card fan are working properly.
Check your video card power connector and make sure that is plugged in. Consult your power supply documentation to determine it's capabilities.
P: System freezes, reboots, or displays poor grafic performance.
A: Make sure that your new hardware it's compatible with the rest of the system, in this case the video card has to be compatible with the motherboard and power supply.
Unninstall your old graphics card driver before installing a new one. Also download the latest drivers for your graphics card. Both Nvidia and ATI offer drivers for all their graphics cards; simply visit their support websites and download the driver you need. For best performance in gaming, also download the lastest version of DirectX from Microsoft website. Installing the latest driver can also resolve graphics issues that appear after installing Windows XP Service Pack 2.
Install the latest chipset driver for your motherboard. The chipset on your motherboard is the "bridge" between CPU and graphics card slot. Find out your chipset driver by checking your documentation or go to Device Manager - System Devices. Your chipset should appear as multiple entries in the list (like Intel 945G/GZ/P/PL Processor to I/O Controller - your chipset is Intel 945). Download the drivers from manufacturer's website, install them and the restart.
Try resetting Bios to default by entering the BIOS menu before startup.
Video troubleshooting
If your monitor in blank, make sure your computer is running by listening for fan noise and checking the LEDs on the front panel. If your PC won't boot check my earlier posts about startup problems here. Check your monitor power cable and video cable connected to the graphics card.
Press the power button on your monitor to see if it receives power. Then check brightness and contrast buttons to see maybe they are turned too low or too high. Try another monitor if you have a spare one. If the other monitor works, you know your problem.
Next check your Graphics Card. I will refer here to a standalone graphics card. To inspect it shut down your system and open your computer case. Make sure your video card is fully inserted in the slot. Take the card out and clean it with a soft brush (a makeup brush from your girlfriend if she doesn't mind). PCI express slots have a lever that you have to move in order to pull the card out, so be carefull. After you clean it, reinsert the card back to it's location, screw the bracket into place, reconnect video cable and restart the computer.
If this doesn't work use another video card to test your system. If windows starts with that spare card, you have to get it fixed or buy a new one.
How to remove a virus, adware or spyware
If you suspect that your computer is not working right because of a virus or your antivirus program has even detected one, you should proceed to remove it from your system.
Back up your data first. Because you can't know from start what virus are you dealing with, the best thing to do is back up your important data to a CD, DVD or USB stick. After you've done disinfecting your computer, make sure you scan your backup removable media for viruses before you copy it back to the computer.
Update Antivirus as soon as you done backing up your data. If you don't have an antivirus program, go online and get a trial version. Personally, I prefer Kaspersky, but any other known antivirus that offer a trial, like Symantec's Norton Antivirus, McAfee or BitDefender will do. Even if you've just installed the antivirus program, your virus database is obsolete.
Some antivirus publishers offer online scanning tools free of charge. But be carefull to choose a legit and known site to do that. Don't google it and choose a pop-up that sais "Scan your computer free" because you may get another virus or malicios software.
Scan your computer. Open your antivirus application and start a deep system scan. Your antivirus will search into your operating system for programs that match virus signatures from it's database.
You can do anything else while scanning is made, just minimize the program and do your usual tasks. Scan times may vary from a few minutes to up to over a hour, depending on the computer being scanned.
After the scan has finished and results window has appeared, you are ready to remove the virus.
Antivirus programs can automatically remove minor threads and put the unwanted program or file in Quarantine. You can manually delete them from this "vault" where your antivirus puts them, depending of the program that you are using.
Adware and Spyware programs. Most antivirus publishers don't offer adware and spyware scan. If you've scanned your system and your antivirus has found nothing, you should try antiadware/spyware programs. You can try Lavasoft's Ad-Aware and AVG Antivirus for free.
If your problem could not be solved by antivirus programs or antiadware/spyware you should consider a fresh install of your operating system.
Printer troubleshooting
P: Failure to communicate
A: If your document will not print or the printed page contains meaningless text, the problem may be that the PC and printer are not communicating efficiently or at all. First be sure that your printer is turned on and connected to your computer. Check the lights and display to see if it's online (connected and ready). Check your printer documentation to see how your printer signals you a problem. May be a paper jam, out-of-paper or low-ink. If nothing like this appears on it's display turn it off and then back on. If you are printing from a removable media (cd, dvd, usb drive) try to copy the file to your hard drive.
If your printer works through a specialized connector, such as a USB hub, disconnect the printer and connect it directly. If this works you may need to replace the hub or your printer may not be able to operate through one.
Check your printer status via Start-Printers and faxes. Locate your printer icon and open it. In print queue window look for your document. If there are other printing jobs ahead that document cancel all of them and try printing again. Make sure the status is not paused or stopped. If so, click resume. While doing all that watch your printer for status changes (message displays or led flashing).
Test page. Try printing a test page. Right click your printer's icon, select Properties and click Print test page (General tab). If the test page won't print, your pc and printer are not communicating. Check your printer documentation for printing a self-test page. If this is not working, your problem is about the printer. If the self page prints, you have a problem with your computer or connections. Try changing ports. To do that go to Printer and faxes, right click your printer, hit Properties and go to Ports tab.
Also try reinstalling the software for your printer. Check for updated drivers on your printer manufacturer's website. Hewlett packard, Samsung, Brother and other popular brands have a good support site, sere you can easily download your drivers. Go to printer properties like before, Advanced tab and click New driver.
If this doesn't work, your port may be corrupt. Return to the Ports tab and delete the one that is used by your printer. Restart windows and the system should detect and reinstall the port. If not, your pc may be damaged.
If your test pages print, but your documents don't the problem may be the program you are using or the document itself. Try printing another document and use another program to open and print it. Make sure the Print to file option is not selected.
P: Error messages.
A: There are two types of printing error: windows errors and manufacturer errors. If you dont see your error message listed down you have to check your printer documentation.
"Device is not ready". Windows does not recognize the printer as being ready to print. Check power and network, if you use your printer on a network.
"Incompatible Print Driver". Update your driver. This is caused by an operating system upgrade.
"Out of resources". This means that you have low resources to print. Insufficient memory or hard drive space can be solved by closing some programs and free drive space.
If everything I said until now didn't work for you, try using another PC. If it's still a fail contact the printer manufacturers or buy a new one. If this works, try reinstalling windows.
Computer statup troubleshooting
P: My computer has no power and won't start.
A: As serious as this problem may seem, a computer that won't start isn't all that serious. Check the power cord and see if it's loose at both ends. Check if your computer power supply has a switch and make sure that is switched "on". Also make sure the surge protectors, outlets and fuses are ok. Try pluging your computer directly into the wall outlet.
Your next step is to open the computer case and check the motherboard. Most motherboards have a led inside that will stay on even if the computer is turned off. If this led is on that means your computer receives power and the problem is elsewere. If not, check the power supply with a power supply tester, you may have a dead PSU. Also verify your motherboard for unplugged cables, make sure that the motherboard power cord is attached to it's socket.
P: I have error messages before starting windows.
A: Most of the error messages indicate a minor problem. If an error message offers Last Known Good Configuration or Safe Mode, the error is about windows' startup. Selecting Last Known Good Configuration will solve your problem. This is mostly caused by a hardware change in your system (a hardware item recently connected).
If you got an error message regarding your keyboard check it's cable and make sure that all the pins of the connectors are straight. If the issue persists buy a new one. I recommend Razer in the gaming area and Microsoft for other users.
Memory and parity errors indicate problems with your Ram memory modules. Remove them and try changing their slot or check the slot for dirt. Clean it with a soft brush and vacuum, but be carrefull and don't touch anything with the vacuum cleaner.
If the computers starts you have solved the problem, if not, you Ram module can be damaged.
Unexpected Interrupt in Protected Mode. This message appears when you've made an incompatible change to your BIOS(Basic Input/Output System). You solve it by entering BIOS and look for a way to reset BIOS settings to default.
NTLDR missing. NTLDR (NT LoaDeR) is the first file loaded in the boot process. Restore the file from Windows XP CD-Recovery Console. At the command prompt type "cd.." and press enter. Write the letter of your CD drive and press enter. Type "cd 1386" and press enter. Type "copy ntldr c:" and press enter. Type "copy ntdetect.com c:" and press enter. After copying type "exit" and press enter. After restart, your pc should be ok.
P: I get a beeping sound from my computer
A: When something goes wrong that causes BIOS to be unable to start, it uses specific sounds to give you a clue of what is going on.
New hardware.Check your hardware to see if everything is in place. Reboot your computer. If the beep continues, remove the new hardware and see if the beep goes away.
Beep codes.Ussually your motherboard documentation contains information about your BIOS version. Knowing that, you can check on the internet for the problem behind the beeps.
P: Error messages after windows starts
A: Check windows utilities. Press F8 before windows boots (check your documentation if this key doesn't work) to launch Windows Advanced Options Menu. Select Enable Boot Logging. When windows starts, go to c:/windows, open ntbtlog.txt and browse for error messages. Work in safe mode to repair errors.
Safe Mode. Select it from windows advanced options menu. Safe mode loads windows with a minumum set of drives and processes to be more effective. Check your pc for malicious software using an antivirus scan and check your hard drive for errors and bad sectors. Reboot your computer. If that didn't solved your issue go back to safe mode and unninstall any program you recently added to your pc. Also try to restore your computer to an earlier date. To do that go to Start-All prgrams-Accessories-System tools-System restore.
Last Known Good Configuration. If you can't do a system restore, try to select Last known good configuration from the Advanced options menu. Your registry and device drivers will roll back to the last setting under which your computer had no issue.
Windows CD. Use it to repair your operating system. Press any key to boot from it (if options doesn't appear go to BIOS and set your CD drive as primary boot device). When prompted press enter to enter windows setup. Press F8 to accept license agreements and then press R to repair the selected windows installation when prompted.
Recovery console. Boot fromk your windows cd. Press R from the welcome screen. Enter your administrator password if required. At the command line, delete your existing registry files by typing "delete c:\windows\system32\config\system". Repeat for times, but instead of "system" enter: "software", "sam", "security" and then "default". Now copy the old versions of those files by entering "copy c:\windows\repair\system c:\windows\system32\config\system". Repeat four times with same words as I mentioned earlier. Type exit, press enter and reboot.
P: My computer is restarting
A: Hardware. Your computer may fail to boot if your memory modules are not well placed in their slots. Check the clips at both ends of the memory module. If the clips are not closed, press that end of your memory module and close it. Restart your pc.
Check all the power plugs from your power supply to your motherboard and other components to be connected properly. Listen for your hard drive to start spinning. If you don't hear anything, it may be faulty or disconnected from the motherboard. Also, be sure the fans on your motherboard or graphics card start spinning. If any component don't seem to responde try replacing it.
BIOS. If settings in BIOS have changed or become unstable, you have to reset the settings. Restart and go to BIOS. Look for options like "Load Defaults", "Load optimized defaults", "Load fail-safe defaults". First try loading Optimized Defaults, and if your computer fails to start again, load Fail-safe Defaults. If you don't have a choice just load the default settings.
Sometimes your computer won't leave you to access BIOS. In this situation, you have to reset BIOS manually. 
The CMOS (complementary metal-oxide semiconductor) memory retains your computer BIOS settings and ca be reset by moving a jumper on a three-pin header on the motherboard and, after a short wait, moving it back to it's original position.
Restarting without error messages. If your system is infected with a virus or malware you may never see an error message that can provide clues. Windows is set to restart any time it encounters a critical error. By disabling this can show your the error messages. To change that, boot your computer into Safe mode and access Control panel, click System icon, Advanced tab and the Settings button from the Startup and Recovery section. Next deselect Automatically Restart checkbox in the System Failure portion. Click ok and again ok. Restart your computer. Now when your computer encounters an error, it will remain on screen.