I have the same problem, with my computer. Are you using Windows XP? Apparently Microsoft, doesn't know why this is happening to my computer, my video card is GeForce 6800. Is the first time your computer has rebooted? Keep track of how many times this happen's, and let Micrsoft know. If it happen's to other players, they will problably make a patch to correct this problem.
Try Fixit Utilites 5, after I installed this program it fixed alot of problems. I played for about four hours, and had "no" problems. And you know what is funny, Microsoft had no idea what was wrong with my problem. So far, no problems as of yet.
FixIt Utilities 5 or (6)new version.