PC keeps rebooting while virus scanning

I think I got a virus last night as this morning my pc keeps rebooting after a few mins. spyware scan(using spybot & adaware) spotted nothing. I use avast anti-virus. On a std scan it picked up a virus and deleted it. Prob kept happening so I tried a thorough scan. I tried this with windows in normal mode and after a while(scan seemed to be going ok) windows rebooted. So I tried this in safe mode and same thing happens. I want to do a thorough scan to make sure I don't have any more viruses.

Any ideas?