Windows XP shuts down when gaming

[MAKO]

System AMD Athlon XP 3500 2.5GB, Gigabyte GA-K8U-939 MB,Nvidia Geforce 6600 AGP 8X, Maddog sound card, 1GB DDR 400 Ram.
Black Hawk and Battlefield 1942 used to play fine. Now within 30 sec of game play the computer shuts down and re-boots itself. Also does it with two new games I tried loading. Updated video card driver and did Windows XP updates. Other than this problem the computer runs fine. Suggestions please.
Also can someone tell me how to check the ping time?

[IM back!]

I had that on multiple pc's it usualy means one of the folowing:

Overheating, what are the temps on the system?
Faulty PSU (likely), try anotherone if you have a spare.
Faulty ram, downlod this iso: http://www.pclinuxos.com/index.php?option=com_ionFiles&func=download&Itemid=34, burn it and run memtest86+
Faulty video card, only thing you can do is replace it to find out.

[MAKO]

Thanks, swapped out the video card with another that I had and that seems to have solved the problem.