System restart when I....

[madrattlesnake1]

OK I'm not sure if this is a Steam problem, a video card problem or some other problem. Ok this is what happens, I play HL2 or Counter Strike: Source. I'm able to play no problem, nice frame rate, everything fine for about 20 minutes. Then, all of a sudden, after 20 minutes my computer jsut restarts out of nowhere. Did this happen to anyone before? Can this be my video card or its drivers or is this a Steam problem? I jsut updated my Catalyst drivers. My video card is the 9800 pro 128mbversion. Also, another thing is that when that happens it sometimes asks me to redownload cs:s. I installed it directly from the DVD the first time around. I also tried to play HL2: Deathmatch and it says I must redownload it even though when I isntalled the game I installed that component. Do these problems have anything to do with each other? Anyways, any help would be appreciated! Thanks!

[meganox]

if you get system properties up in the windows control panel, click on advanced, startup and recovery and deselect the 'automatically reboot' option. you'll now get a nice blue screen of death instead of a reboot. won't do anything for your problem, except the error message it gives you might be some use when you post (write it down exactly). i would search the forums and read as much as possible, i think lots of people have had this problem, but i'm not one of them