You could run HijackThis (link above) and post here the log (if no confidential information is disclosed and if forum rules allow such a non-lfs related activity in the technical assistance section), or take a look at HijackThis (Info on selected item gives you some hints) and concentrate on Search Pages and suspect Browser Helper Objects and Startup Programs. Also, with Hijackthis, you should check in the misc tools section if the hosts file has been modified: normally the only uncommented line would be the one referring to 127.0.0.1 localhost, by the way if there are some particular (good) reasons the hosts file could be used to resolve something else than that.
As usual I suggest to take care: crippling your machine with HijackThis is nearly impossible, but if you disable things you're not sure about you could easily end up reinstalling some apps or having some trouble. If you're not sure, ask.