Your system is very likely infected with some type of Spyware Browser Hijacker.
Click here to have a simple test run on your system. This cannot find every problem, but it will find the most common ones.
I'm going to assume that you will need to go further to solve your problem. There are three Anti-spyware utility programs that I like very much. They are effective and free of charge. To read about these (Spybots, Ad-Aware and most especially SpywareBlaster), go to my page at
http://dwightblackburn.com#spywareIf you need assistance with how to run these programs, please post again. The SpywareBlaster is a prevention tool. It recognizes known threats and prevents them from running. The other two are detection/removal tools. They find and then remove known problem files.
And make sure your Windows Operating System has the latest updates. If you are setup to update automatically, you can click the Start menu / All Programs / Window Update (or Microsoft Update). Download and install all updates.
A note about Automatic Updates: Some virus/spyware will disable your auto updates functions so it would be a good idea to manually run the update process to be sure you're actually getting all the new updates.
You should also run a virus check with your updated Antivirus program. If you don't have one installed, you can get an excellent one which is free for home use.
This page will show you where to find the free AVG download. Be sure to uninstall any old antivirus programs before installing the new one. Instructions for this are on the page.
Finally, once all of this is done, and if the problem is not completely solved, you can download and run the HiJackThis program (from the Spyware section on my page). Run a scan and then post the scan log here in a new reply message. Be sure to run the previous steps first. Be careful with the HijackThis program. Don't remove anything unless you're sure about what you're doing. Only run the scan and then copy/paste the log page here.
Dwight