run cwshredder to see if it was a hijacker you can also get a different adware remover
The shredder is an awesome tool, granted, but it only fixes CWS hijackings....They are not that common, maybe 25-35% of the logs I examine. And the newer strains of CWS don't care if you run the shredder or not..they will still come back, and often times running it in the wrong instance masks the problem, and it will just re-occur again. I've just had a few particularly tough lessons on that. Thankfully some people that knew more than I were able to bail my happy a** out.

With all due respect, you're playing horseshoes with hand-grenades.

EDIT: And back to the original topic, IE used to do that to me a long time ago...I never did figure out why. I think I ended up switching to a different browser...so I'm not much help there, am I