I would take a look at browser's toolbars and extensions to see if there is something visible that you haven't installed knowingly. And would also do a scan with Superantispyware Free and Malwarebytes Free besides your anti-virus program to see if they find something.
Yeah, same thing happened to me awhile back. It's NOT a site issue; unfortunately, you have an insidious and relatively fiesty browser virus. Get a registry cleaner like CCleaner and go nuts...