Get rid of Norton, it is a memory hog. AVG is a great virus program for free but NOD 32 is definitely the best if you don’t mind a small fee.
Also, you should use a router. This will act as a firewall. You can also use the built in firewall in Windows XP which drains very little.
Run Spybot and Ad Aware once a week. Both are free at download.com
But the 2 most important things you can do to protect yourself are
1. Do not use Internet Explorer, use Firefox.
2. Do not use an administrator account in Windows. Create a new user account that is restricted. This way if you do get hit with something it can’t install itself because the account does not have administrative privelages.