First thing that comes to mind is overclocking. If you've overclocked, either step back on the CPU multiplier or the BUS; or increase memory voltage.
The second theory would be RAM failure. You should download MemTest86+ and allow for about 5-6 "passes".
Thirdly I would suspect PSU. Testing the PSU with a digital voltmeter is the best route, since this concern isn't a "no-boot" issue.
If there is any reason to suspect an OS failure, upon power up, repeatedly press the F8 key, and the go down to "Disable automatic restart on system failure", and then press the Enter key. If this is an OS failure, instead of the rebooting, your system will halt on the Windows Stop Error, or more commonly referred to as the "Blue Screen of Death".