This is a hardware problem.
First get a memory checker and run it in its most thorough setting (the timings of your memory may not be correct).
Next check the power supply (it may be too small, should be at least 450 watt).
Finally it can be a problem with the hard disk or graphics adapter.

Altough unlikely it could also be a problem with one of the drivers (make sure you have the latest released drivers [no beta versions] ).

Good luck, go to the computer store and have them fix it.