Ok, here goes:
You do not need AntiVirus software. You do not need AntiVirus software. You do not need AntiVirus software. You do not need AntiVirus software. You do not need AntiVirus software.
Hope this makes it clear. Yes, use antivirus software for windows (I've got viruses and trogans on Windows only in the past)...but NEVER on a mac. I've had my mac for 3 1/2 years with nothing and it works as good as the day I bought it.
You wouldn't get anything anyway, but if you want to be really careful, just do as Knackers said and don't download e-mail or programs from unknown sources.
If you still want to use antivirus software, go ahead but it's a complete waste of time. And if you're paying for antivirus on a Mac, you're throwing your money directly into the garbage. Put it this way, if there ever is a virus on a mac, your virus protection won't work anyway when it first comes out because the authors will have to patch it first.