Could be anything man... your graphics, processor, RAM, OS, Hell it could be that buggy game(assuming its the game, been reading about the game lots of peps complaining, dont know it its just the pirates or even the legit users thou)
Is the PC in a re-loop of restarting or you can go to windows?
Can you Play any other games?
I can attest from personal experience that the legit/retail(steam in my case) game is NO WHERE NEAR as buggy as the 0-day game was. I have played the game for hours, and have yet to run into any of the bugs from the 0-day.
Anyway, like vann said, it could be any of your hardware, your OS, or any of the software you have running in the background. The best way (imho) is to run the game with as few background processes as possible after all hardware components have been reset to stock speeds (if they had been overclocked previously). Also, you might want to update your graphics drivers, lower the settings on the game, or even try to run the game on a different OS in the same box.
EDIT: detobi reminded me of something. When you are having problems like this, you always want to double check that ALL YOUR DRIVERS are up to date, as the problems you described could easily be caused by faulty/old/corrupted drivers.
[Edited by 7ru7h, 9/19/2009 10:57:41 AM]