You may potentially have spyware, trojans, or viruses on your computer. Make sure your Norton Antivirus virus definitions are up to date and scan your computer at least weekly.
Here are three FREE anti-spyware programs, if you only use one, I'd recommend the first one, but it is best to use all three to be completely safe:
http://www.microsoft.com/athome/secu...e/default.mspx http://security.kolla.de/ http://www.lavasoftusa.com/software/adaware/