I am under the impression that while Macs are immune to PC viruses, we can still pass them along to others. The MS Word Macro virus is also cross-platform. With Norton and a bit of common sense, I have been able to warn people in newsgroups that a posting was infected. I have also received infected documents from friends who were quite glad to find out they had a virus.

I think all computer users owe it to everyone else and themselves to use anti-virus software. I also do not see how Norton can "mess up your HD".