My PC restarts automatically after one/few hours of gaming!

Hi, My specs are
Intel Core 2 Quad Processor @2.66GHZ
2GB DDR3 Ram
NVIDIA 9600GT
1TB HDD
and using Win7 x86

My PC, though on the low end, performs quite well and I never had problems with gaming either, every game usually runs with medium to high settings.

My problem started with Hitman: Absolution, it even runs smoothly on high settings but I played it on medium for the sake of my GPU, whenever I play it like an hour or so my PC suddenly restarts(no BSOD). The problem still persists after I uninstalled it and played L.A. Noire! I didn't overclock my CPU or GPU whatsoever, I tried re-installing all the drivers, set all my BIOS settings to default, checked the cpu temp and all seems normal.

AND BTW, I played L.A. Noire 3 times before in the past, played Max Payne 3 in max settings, Crysis 2, BF3, MoH:Warfighter, Assassin's Creed 3 WITHOUT any problems! But this thing is starting to worry me now... so what is the problem?
how can I fix it? or how to know if any of my hardware is faulty?

Any help would be very much appreciated, thanks!