I would look in Admintools/Event Viewer, to see if anything shows up, then if anything shows up copy paste into google, as the PC is a complex thing, it may be something you installed resent that is incompatable with one of the game drivers.  I have had an ongoing battle with Cod4, it crashes every now and then, and I have carried out every known fix and I still get the odd crash.

But it maybe down to overheating issue, driver issue make sure you have the latest sound graphics, mobo and bios drivers.