Macs are awful; don't waste your time or money.
I'm running Common Sense 2011 edition. It's cross-platform, so I've got it in both Windows 7 and Ubuntu 10.10.
In my honest opinion, I think anti-virus software is more viral than any real virus. Have any of you ever tried to completely uninstall McAfee? My god, what an endeavor. Clicking the uninstall link in the Start menu doesn't actually remove it; McAfee will live on in secret.
Well, that and 99% of anti-virus software is complete and utter crap. Norton? AVG? AHAHAHAHA good luck removing most modern viruses. Also, false positives are way too common.
For an honest recommendation, I'd have to say Ad-Aware from Lavasoft. I wouldn't bother paying for the full version; just stay up to date, run a scan every once in a while and don't be an idiot (clicking sketchy links on porn sites; downloading a jpg, changing the extension to bat and running it because someone on the internet told you to; etc.).
Alternatively, even though I was talking shit about McAfee and I don't use it, I do have a lifetime subscription to its enterprise edition for free that I wouldn't mind sharing; there's no limit to the number of times I can install it. I think it's version 8.7i. Of course, I wouldn't blame any of you for being suspicious, especially after I just said not to be an idiot.
Your gamble. PM me if interested.