Can I also say that if anyone visits a lot of sites aimed at teenagers (ring tones, free games, music download sites etc) they will almost certainly find they get viruses and other dodgy software installed on their computer.
You say you have Norton, make sure it has a Firewall, and make sure the Firewall is on.
A Firewall is different from an anti-virus program.
A Firewall monitors every program on your PC that is trying to go OUT to the internet, and also any program that wants to come IN from the internet.
Obviously if the Firewall asks if "Internet Explorer" or "Outlook" wants to go out to the internet or come in from the internet you say yes. Once you have said yes once it never asks you again.
But if the Firewall asks can "dodgy software" go out to the internet, or come in from the internet, then you say no. Once you say no it is blocked.
If you run a Firewall you need to educate your son to look carefully when a program wants to go out or come in from the internet, and if he does not recognize the program to say NO (if you decide later you want to say YES it is easy to change it within the firewall program).
Running a firewall is just as important as running an anti-virus program.
Note, you dont run a Firewall in the same way you run anti-virus. A Firewall program should start when the computer starts and just sit there monitoring your internet connection.