It's arguable that by running antivirus software on a Mac you're creating more vulnerabilities without adding any real value.

Run personal systems behind a firewall and don't click on random crap like a drug-addled monkey and you'll generally be OK. Be at least minimally picky about what software you run -- no, Outlook Express is not a good option, and yes, there are still people using that particular plague-ship -- and you are probably going to be fine.

(I've run any number of shoddy-OS systems this way over many years, and the only time I've ever had any sort of problem was my own damned fault.)