my pc keeps rebooting?

My PC continues to reboot. Not all the way through the start up. Just once the start menu and desktop icons pop up it begins to open all the atomatic start ups. then it stops clears everything and does it again and again. just picked up panda antivirus pro 2009 scaned for viruses and spyware. 2 were detected and removed. still continuing. cant get online because of the reboot doesnt allow enough time to get connected to the internet so cant update the antivirus. Can anyone help me?