Are you absolutely sure it's a virus then? If one wasn't found, then it might be hardware or a bad Windows install which causes your machine to reboot just like that, at the same moment every time.
You'll want to back up your files (not programs, not Windows) to a separate HDD and reformat your original one to get rid of Windows.
Reinstall. If that doesn't fix it, it's probably a hardware problem of some kind.
Master of common sense. If you don't like it, stop reading.