PC flashes and freezes.

Hello
Sometimes when windows starts up for the first time (In a day), or when I've used it for some hours:
It freezes up after a flashing, flickering screen. It stays flickering and does not respond to anything. Only hard reset can "fix" it.
Do you guys have any idea what that could be?
I searched my PC for viruses to find none with avast and other searching online scanners.
Thanks a lot in advance.
P.S.My video card is not old.
Specs:
Intel Core duo 2.13Ghz
Nvidia geforce 8600GT with latest drivers
Intel motherboard
Windows XP PRO SP3 Media Center Edition


I did not upgrade or did anything to cause this, as far as I know. I am the sole user of this PC.