PC rebooting when...

Since saturday everytime i try to launch a game, my pc reboots itself when loading the game, i can get through the start menus, but once it is loading the actual level the pc reboots. It happens with all games i've tried (FEAR,Guild Wars & Quake4)
I've checked for reheating but the temps are within range both on v-card and cpu.
I've also tried re-installing directx & drivers as well as the games.

Any one have any ideas what the prob could be?

thanks.

BTW specs:
Athlon2000+, 1gb ddr pc2700 (1 stick512, 2x256 sticks), Geforce 6600GT 128,
SB audigy Platinum, 80gb 7200 HD, Windows XP SP2.