It's really sad that there are this many pages of people talking about anti-virus software on here when it's SO easy not to get them in the first place.
Simply use an operating system that doesn't grant system priveladges to it's user accounts. It's been almost 15 years since Win98 came out and Microsoft STILL hasn't fixed this problem. Simply don't allow the user (that's you) to have write access to system files and viruses are a non issue. Mac figured that out early and it's taken years for the first real mac malware to show up. I once got a virus on my linux machine, it asked me for a password and told me exactly what it wanted to do before it ran. (so I deleted it) It's a simple fix, but Microsoft makes too much money on security tools to fix the problem. They'd just rather take your money for "protection," than make a secure system.
Mac virii exist, and will become more prevalent as market share increases (especially since the processor technology change). Not sure why people buy Macs anymore, given the comparative price. Linux is great for some, but there are still issues to work out. It isn't immune to malware, either. (Look at Ubuntu with adware by default.)
Norton and McAffee have been bloated beyond usefulness about as long as I can remember. I quit using AVG when it couldn't get rid of some things it found. Avast free is great, and I've never an issue using clam-based systems in *nix. Spybot S&D is also useful as an addition.