Again, there is no difference physically between Crysis and other CPU / GPU intensive programs.
If your CPU is getting to 100C, that's pretty hot... now most CPUs can tolerate just a bit more heat than that, but not much more than that I think is the limit for your CPU... though I'm not sure so if someone else could confirm that for me I'd appreciate it. (Or deny it... either way)
Anyways, if you're saying that the system didn't shut down during a 3d model render when it was active for over 12 hours, then It's probably a GPU issue rather than the CPU, cause a lot of 3D modeling programs are more focused on the CPU than the GPU, so therefore your GPU (if there's a problem with it overheating) would not have overheated cause it wasn't being used so exclusively as in the game.
I had a virus a while back that did something similar to this. The computer would be going, just fine... then poof! It'd turn off and I'd have to hit the power button to turn it back on.
I just had a problem with my Sony Vaio (relatively new FZ290) that it would run for 2 hours, then it would blank the screen, and not respond to keyboard / mouse or anything, and you have to kill the power to it. This turned out to be some weird anomaly in the monster we call Vista. Reloaded OS and now it's fine.
If the fan doesn't spin full speed when you're in game, that could be a problem, or it could mean that when you quit the game, the transition is hard for the processor so it goes and speeds up the fan.
As far as underclocking the CPU/GPU, not sure if XP has any options for that.
I don't remember if it did, but i don't think so. Check anyways.
You could see about turning down the settings on your games and whatever else you run. That might help.