Throw your computer out the window, that one is one of the nastiest I've ever had to deal with:
http://www.bleepingcomputer.com/malw...antivirus-2009
NOT legit.
EDIT: Don't just hit cancel or whatever, shut down your browser or end the task in task manager.
Hopefully it's just a popup and you don't actually have it. If you do, God have mercy on your soul.