There could be a number of possibilities. First list your specs and your PSU including wattage.
If you are trying to play a highly graphical game ala Doom 3 and you blast up the settings but you have a weak card or a poor cooling system in your PC the PC will automatically shut down as a defense mechanism when it feels like it is overheating. Same as a human body when they drink too much...it shuts down to prevent more consumption which would result in death. If the PC kept going eventually your GPU will fry and who knows melt your mobo or worse your PC will catch up in smoke.
If you have a high end GPU don't run it with a PSU under 400 watts to play it safe.
It could be your PSU is defected.
Or it could just be a driver issue and therefore a driver bug. In that case you have to wait for an update but usually those dont shut your PC down but just make game crash.
Or it could be another part of your Pc overheating like your CPU which then you could be in big time trouble because that will melt your mobo if not cared for.
Like I said a number of things. List your specs, the game it happens, and the settings and people can localize your problem and help you better.
Well I reinstalled my driver then I played need for speed underground 2 for about an hour and a half and no problems so maybe you were right. But I'm not gonna get my hoped up just in case it's still messed up. Even though, Thank You.
what brand is your power supply and once again, tell me how many A(mps) is under 12V (this number is above 10, not the other 12V which is a low number)
also, check your cpu and gpu temp under load to make sure its not overheating