I've seen this happen before on other vBulletin forums. I personally haven't seen it here yet! If I remember correctly it took "the Boss" to get it cured.
For those who are experiencing these weird malware/spyware/virus shenanigans... Just out of curiosity, what browsers are you using? IE, Firefox, Chrome, etc? If you have more than one browser on your system, do you see the same behavior in all of them?