Forum Moderators: phranque
Control Panel > Administrative Tools > Computer Management > Event Viewer
Check the system log (and the application and security logs too if you are so inclined) - that should contain the pertinent information.
I don't think this is a virus, although I guess I should check. The computer has only rebooted twice like this, although I was only present for one (and at that time learned that "reboot" is the wrong word since it really just turns off without notice).
From a low kind of whine that emits from the computer when this happens, I'm guessing there's an error with the CPU or a hard drive. I don't like either answer, but I can't think of anything else that might make a noise like that.
I would either take off the side cover and watch it until it shuts down, or locate one of the motherboard software programs that monitor the temperature of your CPU.
If it's shutting down really fast, you may not have enough time to install and run the motherboard monitoring program.