This kind of problem is caused either by compatibility issues, power shortage, viruses and spyware, or any combination thereof.
I would first run virus and spyware scans. If you still have a problem after that, you should update your video and sound drivers. If you still have a problem, try reinstalling the game. If that doesn't work, you might want to check the amount of power needed to run your computer under full load; if it exceeds the amount the power supply is able to deliver the power supply will restart your computer.
If you still have a problem, try taking out unnecessary cards in your pci slots, and if you have another video card or onboard video you may want to remove or swap that, too. If this solves the problem, put the various hardware back one by one, and if the problem returns after adding a certain piece of hardware, you've found your compatibility issue.
If all else fails, write all your music and pictures and game files, along with anything else you want to save, to DVDs and reformat your hard drive using the Windows CD and reinstall Windows. Don't immediately reinstall all your software. You want to install all your software slowly in case the problem returns, that way you know what was causing the problem.
Hope that helped