As noted by others, a spontaneous reboot is almost always a result of either a hardware failure/overheat, or a supressed BSOD (Control Panel->System Properties->Advanced Tab->Startup and Recovery Settings and uncheck 'automatically restart').
I did that when I had a similar problem when running COD4, my box still went down. Not saying it doesnt work, just didnt in my case.
On the software side of things, here is another thing you can check. Check your event log for any errors corresponding to the time of the reboot. chances are, if theres an error, it will be recorded right before the crash. I used to have an ATI graphics card, now I am using an Nvidia. I did the whole uninstall thing when I removed the ATI card and thought I had everything. After crashing 4 times in one night, I checked the log, it showed an error when trying to load the ATI crossbar something or other, all 4 times! These errors corresponded with times of the crashes, so I went back and did a thorough removal of all things ATI and all has been good since.
Good luck to you... this is the worst type of trouble shooting to perform...due to the immediate and unpredictable crash, lack of warning signs prior to the crash, lack of clues after the fact and the abundance of potential causes. Like Kryo, At first I thought this was likely a hardware issue and was going to start replacing parts, starting with the PSU. I just happened to check these logs as a last resort, maybe a clue to what device was failing. These logs will be one of the first things I check form now on....
