Who uses anti virus software... I have never used it, and I have never gotten a virus.
brianhos is right. Anti-virus software is a scam. Since it doesn't work if you feel you must have it, use one of the free ones like AVG.
The best setup?
One, have a router. Never connect a computer directly to the internet.
Two, run a basic firewall. Even the default Windows XP firewall is fine.
Third, NEVER open an attachment to an email. Even from your mom.
Fourth, stop surfing porn.
Fifth, stick to the lighted areas of the internet. ie, don't follow links on MySpace...
Sixth, ALWAYS surf the internet as a limited user, never as admin!!
Seventh, use OpenDNS.com to block phishing sites (can't recommend this enough)
There are a couple more things I can't think of right now... Go to grc.com, and listen to the Security Now podcast if you want to learn more.