Have you added or attached any new hardware to your pc. Via USB or somthing?
Try rebooting the PC with nothing on/attached then 1 by 1 attched your mouse, keyboard, printer and what not and see if that helps .
Ok next one .....
I had a problem simular and it was too much heat in the PC after placing a NEW G card in there caused the PC to reboot during games. Try removing the lid/s and then reboot. Your PC could not be cooling down properly. hope 1 of these helps let me know.