you've got some kind of spyware or other malware... probably coolwebsearch or some other that I cant think of the name of right now.
what webpage does it reset as your homepage?
you should go to downloads.com and download lavasoft adaware and a program called spysweeper. then, update the definitions for both programs and run them one at a time. This will get rid of most all types of spyware- some of these nasty little buggers arent so easy to get rid of though, they repropogate themselves on reboot. So, if the above doesn't work for you then you could download a program called hijackthis- what this does is give you a list of questionable registry entries, NOT ALL OF THEM SHOULD BE DELETED. If its not at
downloads.com you will find it doing a google search.
If you get to the step where you need to download hijackthis you can feel free to email me the results and I will look through it for you and make some suggestions if you like.
If you're able to ascertain exactly what the problem is there are also specific programs to do away with the spyware that the above programs (spy sweeper, ad aware) won't. One of these is CWSshredder (or something like that) and it is specific to coolwebsearch.
I hope this helps. and I'm sorry its all disjointed, its time for me to sleep.
edit: Prevention is better than the cure. once you get this problem ironed out its a good idea to run adaware and spysweeper on a regular basis and dont download from any of those gray pop up boxes that show up on webpages sometimes unless it is signed, verified, and from a company that you recognize.