I had the same exact thing happen to my system while playing KOTR2. I have a 6600gt, athlon 64 3200, 420w power supply, etc., etc. The game ran like a champ the day I got it and I played for 3-4 hours. The next day the game crashed on start up and forced a restart for the entire system. After trying every solution given on this board with no fix I decided to call Tech support. (btw, no other games would work either, nor could I run any benchmarks just regular desktop things and the net))
Anyhow, tech support seemed to think that it was a bad ram chip. Sure enough, it ran like a champ again oncce I took out one of the sticks, they are sending me a new MOBO since they seem to think that the slot went bad. Anyhow, I figured the problem was solved, until 2 hours later when it started doing the same thing again. Now I was starting to think that maybe it was the video card (XFX 6600GT) I called them and they told me to try the BETA drivers.(no change) I tried an old video card, and it worked (just way too slow, an old gf4mx400) XFX told me it was a bad power supply. I called my comp tech support and told them what I was told by XFX. They said they wanted to replace the MOBO first and go from there.
In the mean time, I removed the one ram chip (now running only one 512 3200 400mhz) and took out all the case lighting, and removed all non essential USB devices. KOTOR 2 runs now but who knows when that may stop. It seems to me that it could very well be the power source since my problem seems to be solved when I remove as many drains to the PSU as possible.