PC crashes can be caused by many different reasons.
If the crash occurs always with the same error message: Write down the error message and google for it!
If that is not the case and the problem occurs only during graphically demanding operations of the PC like playing action games, video playback or browsing flash animated websites, check the graphics driver of the PC graphics card, if you have the latest version of the driver. If not, install the latest driver for the graphics card.
If it is always changing error messages (or crash with no message at all) and the graphic card driver seems to be OK, the next thing I check is the RAM modules. For that I burn a booting CD with a very small memory test program which starts after booting. Do you know how to burn an ISO image file to CD? You can download RAM module test software here:
http://www.memtest.org/#downiso
The software needs the first 0.064 MB of the RAM modules to be OK (otherwise the software test is not possible), but if you have 2048 MB of RAM installed, you can easily test the remaining 2047.936 MB if they work OK. Let the test run for half an hour to one hour or so, if the error count stays at "0", the memory modules should be OK.
Crashes can also occur with a faulty power supply: Either your mains power line from the generating station has problems, or the power supply of the PC has become faulty. If the voltage problem is from the generating station, it should be gone after some hours. If it is from the power supply of the PC, things will get worse after time.
It can be very hard to find out what causes the crashes, if the crash does not happen always with the same error message but there is either no error message or the messages are different from crashing to crashing. Except it is the RAM module, but that can easily be tested (see above).