Spyware acting as Windows – Please Help?

There is this yellow dialogue box that keeps appearing every 10 secs that says “you computer is infected! ..windows will now install antispyware for you” and then it take me to a site that asks for money to fix this problem” i thought i solved this by using spy doctor and it was gone, but whenever i restart my computer it keeps coming back. And each time i scan with spyware doctor this file “Adware.Agent.ZO” keeps coming back. Even after the spyware deletes it…whenever i restart its there again. Please help.
thanks but how do i put my computer on safe mode

Malwarebytes is on of the few programs that can remove this type of infection – http://www.malwarebytes.org/mbam.php

The other is – http://siri.geekstogo.com/SmitfraudFix.php