Back to the actual question--I've had Norton for years and it does a pretty good job. I'm always wary of "free" programs as I believe they are holding back a little something.
In magazines that have run tests of these anti-virus programs, they virtually all miss something or other. None are perfect. I have Norton (for virus), Stopzilla (for Pop-ups), Ad-AWare(to remove anything that gets on) and Zone Alarm (primary Firewall)--and once in a great while something STILL gets through.
It's a never-ending battle..