Sorry, but we'll need MUCH more information than this if we're to work out what the problem is! What OS are you running? XP? Does it regularly do this or does it seem like it's random? Does it do it when the system's under load, or even when it's idle? On the blue screen it should give you an error (or STOP) code, what is this code? Does it vary?
A couple of things to try generally would be to run chkdsk (chkdsk /f C and run memtest (google around to find a CD image you can boot.) You'll probably need to leave memtest running for a good 24 hours to get a useful result - then post back and say whether there were any errors