My computer has been crashing or rebooting at random points in any video or game. The first thing I checked was if I had the latest drivers for my ATI Radeon 9600XT. I had the latest drivers, so I ran diagnostics on all of my computer's components and nothing found a problem.
In my event log, there were plenty of entries for "ati2mtag" which ATI claims wouldn't force the computer to crash. After reinstalling Windows XP with SP3 the same problem occurred. I looked up the problem and apparently quite a few people were suffering from it.
Following other peoples suggestions, I tried drivers from Omega Drivers. My problems ceased instantly. It's sad that ATI can't support their own hardware as well as a third party. It's nice to have people like this around to help.