I take it you have scanned your whole system with Norton and it hasn't found anything?

If so you may want to consider getting rid of Norton and installing AVG or Avast Anti Virus programmes....Both are free and once they are installed they will scan your sytem to pick up and deleate anything that is causing a problem...Or if you are using the paid version of Norton...Why not try copletly uninstalling Norton and then re-installing it again.....
You havent mentioned if you have a firewall installed on your machine,If you havent please install one as they are a great way of keeping out all the shit from ever gaining access to your system....One of the best is Comodo which is also a great freebee and will not let anything on your system unless you authorise it to do so....
Another thing you may want to try is deleating all of your cookies and temporary interet files before installing Spybot S+D which makes sure that most of the spyware that is available cannot get on your machine.....
Finally you may want to install Adware 2008 to keep al the adware off your machine

Heres the links to all thease free programmes.....


Firewall

http://www.filehippo.com/download_comodo/

AVG
http://www.filehippo.com/download_avg_antivirus/

Avast

http://www.filehippo.com/download_avast_antivirus/

Spybot S+D
http://www.filehippo.com/download_sp...earch_destroy/

Ad-Aware 2008 7.1.0.1
http://www.filehippo.com/download_ad-aware/