Okay, there seems to be a few Mac owners that are worrying themselves silly about trojans, worms and other virus infections.
The answer is don't.
Simply make sure you have your in-built firewall turned on along with the firewall that's configurable within your router's settings. That's the main done already!
Secondly, always make sure you run Apple's Software Update as often as possible. This can be set to run automatically at regular intervals.
Next, stay clear of dodgy websites like Pirate Bay and many others. If you want something - save up and buy it! (or google for a freeware alternative).
Why take the chance that your stolen download, from a torrent server or other file sharing website, is the legit thing? The recent issues with people ripping off Apple's excellent iWork is proof of this but the sad thing is that it's such an affordable purchase! Hardly M$ Office or Photoshop and there are even FREE alternatives like Open Office.. No excuses!
In fact, never install something that you didn't personally download yourself from a legit site like MacUpdate.com or direct from the author's website!
It's all about common sense folks. You've already made the right choice by not buying a Windows based PC so why risk deliberately infecting your Mac?
IF you must install virus protection software then I'd personally advice that you do not consider Norton AntiVirus. It'll do a great job of slowing down your Mac and others have complained about conflicts and crashes. Intego's VirusBarrier is a better choice BUT even this slows down your Mac.
Interestingly, I scanned my hard drives recently and we're talking around 2 million files spread over 3TB's of disk space dating back from 2003. Zero viruses found! Think about it...
However, I do recommend you try out the freeware program ClamAV as it's a cool little app that doesn't slow down your computer too much. Though please make sure that you read all the information associated with it first.
I have ClamAV installed now...
...of course, it also has found zero viruses! ;-)
Bottom line, if you're gonna be an idiot and download illegal software or visit dodgy porn sites then what do you expect is gonna happen to your lovely Apple Mac? Happy computing!