Remember when Ad-Aware wasn't a huge bloated mess? Might still be worth a try (free edition) if nothing else turns up the solution.

Hopefully it's just a browser-level plugin. If it affects all browsers it could be something installed as a proxy and might even show up in Windows Internet prefs.

Doug, are you still running Windows XP? Windows 7 should in theory spot stuff like this with its malware "stuff" - I can't remember if it's Defender or the other thing I can't remember the name of right now.

You should try Ubuntu.