Sounds like at some point you or a site without your knowledge has installed a toolbar to your browser. this is not uncommon, what would suggest is getting hold of a copy of "ADWARE 6 S.E." this will install a spyware scanner that will run in the back ground. that will prevent this event from happening again. and will tell you when things are trying to do nasty things to your computer.
that covers prevention, now to cure the current fault.
go to this site :
http://www.intermute.com/spysubtract/cw ... nload.html
and click on the download button at the top right. this will install a scanner designed to locate and remove ANY browser take over programs that may be lerking in the registry or the browser itself. Open the program once downloaded, click AGREE to the terms, then click the FIX button. this will then scan your system for any verient of CW resident programs.
once found and removed. REBOOT, and run the scan again, to make sure nothing has re-asserted itself.
you now need to address your firewall / virus scanner / spyware protection needs, or this will continue to happen.
Cheers.