PC Game Crash - Auto Reboot

Heyas, looking for help.

I got a problem with 2 games that crash, more like they actually reboot the computer instantly.
It use to be just The Witcher at 1st, but recently tried for the 1st time to get the Sims 2 and the same thing happens.
I changed all options (trust me) of both games, update graph drivers, directx, reinstall the game, clean registries, clean spywares/virus, control temperatures for heat.
In the end, when the game starts (no problem at the menus part) a 1/2 second later it freezes, and for a second it shows the game start view with sound doing a frozen loop. Then Bip, restart.

Note: I can play games like GTA San Andreas, C&C 3, Oblivion, Overlord, Hellgate London, Crysis, Neverwinter 2, etc... and no crash what so ever.

I did have 2 crashes/reboot playing Medieval Total War2, when spinning the game view towards a city during a battle.

I thought the problem was that The Witcher was in someway a 'heavy' game but I can run Crysis and not Sims 2!?
Any ideas? Thank you for any help.

System:
Windows XP SP2
Directx 9C
Intel Core Duo CPU 2.66GHz
RAM 2GB
NVIDIA GeForce 7300GS
Realtek HD Audio output

Nope not from any vendor. I guess I can try get a different power supply if I can't figure out a way for a while. Thank you