You are infected with spyware.  Download, install, update, and run the free versions of Ad-Aware and Spybot and let these programs remove all of the garbage that they find.

If these programs don't solve the problem, go Here and follow the directions to download and run the analysis tool called HijackThis. Generate a log file, then open it and copy and paste the text of the log file in a message in the HijackThis forum - http://forums.us.dell.com/supportforums/board?board.id=si_hijack where a HijackThis expert can offer advice on how to fix the problem.

Steve