I always differ with Jimoblak on this one.
To date, there has never been a workable virus/worm on OSX. While “theoretical” ones have been reported, none have ever been seen to actually work and spread in the wild.
The same goes for other malware. There are constantly reports of “holes” in OSX, but none has ever been reported to work in the real world.
Macs get FAR more attention in both the mainstream and high tech press than their market share would justify. You can be guaranteed that, if real virus/malware problems were shown in the wild, you’d be reading about it very quickly.
I’m responsible for a small fleet of macs in a marketing firm. We have been using them for 15 years. We have never had any sort of protection software installed and have never had any problem.
Just make sure that you only enable what you need on System Prefs/Sharing – under both Services and Firewall. The only thing we have enabled is Personal File Sharing.
Using OSX 10.3.9 on a G4