This is a common tactic with many viruses, trojans, and worms. Frankly, to fix it or stop it, you'll have to find which exact infection it is, and then do whatever it takes to scrub it off your system. If your current anti-virus doesn't detect anything, then get another (I recommend AVG Antivirus...free for home users). You can try some reliable online scanners as well, and when you get a virus name, google it. See if a reputable company (like Symantec, or others) has a removal tool or procedure if your virus protection can't fix or isolate it.
Oh, whoops. Sorry. I guess I misunderstood.