Shuts down whenever I play a game

So I was playing Hitman: Blood Money, and this is where it first started. About the third time I played it, my computer shut down, and not like a normal crash, like actually shut down, removed all the programs, and had the box popping up, now shutting down. And this happens after every game I play. And tonight I was playing COD4 for 3 hours, and after all that time, my computer did it again, this has never happened before, before 2 weeks ago whenever it started. It does not do this whenever I play counterstrike: source. I'm wondering if there is somewhere, where if my computer overheats it automatically shuts down? I respect my computer the way a normal geek should, but it's pretty old and I don't need a new one? But I just don't want it to shutdown anymore, it really cramps my mood. Any help on this would be most appreciated! Thanks My information: Processor: AMD Athlon(tm) 64 Processor 4000+, ~2.4GHz Memory: 2048MB RAM Hard Drive: 110 GB Video Card: RADEON X850 Series Monitor: Plug and Play Monitor Sound Card: Realtek AC97 Audio Speakers/Headphones: Logitech Headset Keyboard: USB Root Hub Mouse: Logitech G5 Mouse Surface: Operating System: Windows XP Professional (5.1, Build 2600) (2600.xpclient.010817-1148)