One of the things I swore I would do when I changed my computer was to NEVER install Norton antivirus on it, even though my ISP provides it for free. It might be a good piece of software for keeping viruses away, however I believe it's like a virus itself once it gets on the pc, like windows always opening up boxes asking you to authorize stuff or warn you. I really, really hate it. I'm sure it slows down the PC as well.
Since I bought my Mac I use the simpler Norman antivirus in XP, since I get it free with my netbank (they must have faith in its security).
Since all my e-mails are written and opened in OS X and most of my surfing, the only chance I could catch a virus would be from surfing on sites that won't open in Mac or by online gaming, but I am glad I have something there just in case.
My ISP also provides free Norton for Mac