Disable your add ons until you find the one that's causing the issue, or reset your browser. I think it's add ons that cause the issue though. I know this has happened before I don't remember exactly what they did before. As far as I remember it was the add ons though.
Figured it was an add on - lol. I had this weird add on that kept giving me advertisements from random words. lol. Still can't figure out where it came from.
It's a trojan that comes with some other downloadable programs, usually when you try to get things fro less than reputable sources. Since it's technically an addon / extension (depending on if you use mozilla / chrome) virus software doesn't always pick it up. It's the same principle as when you download YIM it asks to install the yahoo toolbar - except this program doesn't bother asking.