they are probably programs called spyware. Many of us think of them as viruses, but because they do not actually do any damage to your machine, the anti-virus companies do not include them for deletion. What these programs do is gather information about the sites you go to and pass that info onto their servers so they can direct advertizing directly at you. You probably still want to get rid of them, so check out one of the two following programs
Adaware
Spybot - Search and Destroy
Both are excellent programs that will scan your machine for free to delete spyware. I personally use spybot, but adaware is a good program too.
If you want to double-check norton antivirus just in case, you can go to
http://housecall.antivirus.com/ and run the free web based service. I do this about twice a year, just in case a virus would get in and disable norton
Good luck