I have been using Macs every day of my life since 1984. The last Mac virus I saw was probably in about 1989. Agree with Yenko, stay far, far away from Norton. If you need a disk utility (you generally don't really need to defrag OS X hard drives, either), I would recommend TechTool Pro or DiskWarrior.
EDIT: Now that I think about it, I think I did get a virus in 1993, when I was on AOL 😱 ... but still ... it hasn't been a problem for me in years.