is probably just windows update restarting your pc
that is if you left it on automatic update and install.
Hmm... I left my computer on overnight and it restarted. Event Log doesnt show anything.
I just disassembled my computer just to check if anything was wrong. Only problem i saw was the usb ground etc etc. Thats about it.
I think I know whats causing my computer to restart. It is probabily Daemon tools, but I may be wrong. My computer restarted about 3 times today when I was messing around with it.