If your PC becomes unstable i.e. freezing, crashing, rebooting, then you can take these primary steps to self diagnose the problem.
1. Power off your PC and unplug/remove the HDDS, SSDs, DVD/Blu-Ray ROMs, graphic card (only if you have an APU or onboard graphics), sound card, LAN card and any other cards for that matter. Also, unplug any case except those that are directly cooling components like heatsink/radiator fans.
2. If you have two pieces/sticks of RAM or more remove all except one.
Troubleshooting New Builds or suspected faulty parts:
-Resetting CMOS to Default-
1. Clear your CMOS, check your motherboard manual on how to but the easy way to pop out the battery, pull out your PSU power cable and pump the power button on the casing a few times then put the battery and cable back.
1. a. Get a USB Flash Drive ready and download from http://www.memtest.org the Auto-installer for USB Key (Win 9x/2k/xp/7).
b. Run the setup.
c. Boot to USB (check your motherboard manual if you don't know how to boot from USB device, should be F1, F2, F6, F11 or F12).
d. Run the test for at least 4 passes. The more passes the surer you are it's not the RAM.
e. Do this for each individually stick of RAM.
f. If all the RAM passes to your satisfaction, run Memtest one more time with all the RAM installed.
g. If all RAM sticks passes individually but fail as a group you best bet is to manually set the voltage, timings and speed in the BIOS to force a uniform RAM configuration.
Note: Some motherboards can only run RAM in certain configurations. If your RAM have the same voltage, speeds and timing it is a good idea to run them in dual channel. RAM that do have matching voltage, speeds and timing should be run it single channel. Usually, if you have 4 RAM slots then slot 1 and 3 are dual channel as well as slot 2 and 4. RAM in slot 1 and 2 will run in single channel in a 4 slot motherboard. Running RAM in single channel it is a good idea to set the voltage match the RAM with the highest needs and the speed and timings to the slowest RAM.
-Testing the system-
1. a. Reconnect/install your OS HDD/SSD and boot/install Windows.
b. If there are no problems install/connect the remaining hardware/parts one by one. If the system fails then the newest installed/connected part is likely the fault and may need to be taken back for warranty.
c. Make sure to install an Antivirus software before going on the internet and/or connecting an old media storage device to the PC since viruses might be sitting on your old HDD/flash drive.
1. PC won't turn on even though only there is only motherboard, CPU and RAM are connected.
Check power cable to PSU and check power cable to motherboard. Replace PSU.
2. My PC is beeping.
Check your motherboard manual for type of BIOS it has and refer to this guide http://www.computerhope.com/beep.htm
3. My PC works without the graphic card installed (APU/onboard) but does not with one plugged in.
Check if graphic card fully seated/inserted into slot and check if power cable to graphic card is missing or needed.
4. My LAN/Audio/Graphic is not working.
Use the provided drivers on the CD given with the motherboard/graphic card/sound card. Then download newer drivers from the internet.
5. When accessing/opening files on my HDD my system becomes very slow or crashes.
The HDD maybe failing check the HDD's SMART info. http://crystalmark.info/software/CrystalDiskInfo/index-e.html
6. My HDD light is always on, I can't download or open files and my CPU usage is around 100%.
a. Possible virus infection, if you can download get an Antivirus and if it can help. I have both https://www.malwarebytes.org/ and http://www.360safe.com/ working together.
b. If you can't download or your PC is very unresponsive you need access to another PC with an Antivirus that is up to date and scan your HDD on that PC to hopefully cure the infection or at least identify it so you can download toolkits for it.
7. My PC is giving errors/problems, I think it is the HDD but I can't boot to Windows to install a HDD diagnostic tool.
a. You try Ultimate Boot CD but it requires a little know-how or at least some experimenting to work around the interface http://www.pendrivelinux.com/install-and-boot-ultimate-boot-cd-ubcd-from-a-usb-device/. Or you try (I haven't yet):
b. Bootmed http://www.bootmed.com/bootmed/ make it USB Bootable http://www.myfixlog.com/fix.php?fid=66
c. Lenovo Diagnostic http://support.lenovo.com/en_US/downloads/detail.page?DocID=DS028698
d. Seagate Seatool DOS bootable http://blog.nowherelan.com/2013/04/04/boot-seatools-off-of-a-usb-drive/
e. Western Digital Data Lifeguard Diagnostic DOS bootable http://support.wd.com/download/notes/DLGDiagUSBInstructionsv3.pdf
8. I have reset or flashed my BIOS to the latest version or but now Windows crashes on the startup logo.
Go to your bios and look under the menu window you should see SATA 1, 2, 3, 4, etc. Select that SATA port that your HDD is mentioned, in that menu see if there is mention of IDE or AHCI. Select and choose the opposite. Save and restart.
9. My PC fans automatically start when turn on the power from the main outlet or PC will not power on.
These problems are usually related to the either motherboard’s chipset or power regulation components failing or a bad PSU. Easiest way to figure out which it is to get hold of a working PSU, if the PC still have problems starting without giving beep errors then it is the motherboard. If you recently suffered a lightning storm or blackout before your PC displayed these problems it can PSU and/or motherboard damage from a power surge through an unprotected wall outlet or telephone line.