Now me, I own a DECENT PC running MICROSOFT Windows XP. My computer's a few years old and it can play games like L4D2 on some fairly decent settings. The problem is that whenever I try to play a game like L4D2 or any other games that go along the lines of fairly recent or when it comes to MODs like Goldeneye: Source, my computer unexpectedly shuts down after five minutes of play time. The only games where this doesn't happen are any games that don't seem to take a lot of processing power (or so it seems). I can play the original Serious Sam's with ease; I have no problems trying to play Total Annihilation, Point-and-click adventure games such as the new Sam & Max's play just fine. Though, when I sit down to play a session of Spore, it shuts down five-fifteen minutes after start. No, I don't mean the computer kicks me out of the game, I mean the ENTIRE COMPUTER SYSTEM shuts down all the way down to having to push the power button to turn it back on. I don't know what's wrong and it's starting to become WORRISOME because this has started to happen much more frequently. I've ran just about every single virus scan I have the patience to endure (you know, AVG free, Malwarebytes, Spybot- Search & Destroy, the typical STUFF) as well as used a fair amount of defragmentation software's such as MyDefrag, CrapCleaner, and Auslogics Free edition. I've been using CoreCenter to make sure that the CPU isn't overheating and it's not.

Finally, I've had a friend of mine who's very knowledgeable about COMPUTERS (he certainly knows enough to build one from scratch, given the necessary computer parts) and he tried shutting down one of the power supplies. This worked for some time but now the computer's shutting down whenever I try to play games like Half-Life 2 or Spore. It also seems to depend on the game. Half-Life 2 makes the computer shut down after about three hours of game time and Spore makes the computer shut down after about five minutes of play time. If anyone knows or can at least give a suggestion on what's wrong, please tell me because I'm starting to get my knickers in a twist over this whole thing. I'd be grateful if someone did help me and I just want to thank anyone who does help me.

P.S. Sorry for the wall of text