There’s a reason there aren’t many anti-viruses for the Mac. There aren’t any viruses on the Macintosh. None. I don’t recommend using AV or anti-spyware products on OS X at this time - they’re a waste of money.

If you’re concerned about spreading Windows viruses inadvertently by forwarding email attachments - don’t forward email attachments, by the way - there’s a free open source scanner that does work on the Mac, ClamXav from http://www.markallan.co.uk/clamXav/.

If you use Parallels to run Windows on your Macintosh, install the built-in Kaspersky anti-virus.

If you use Bootcamp you are running Windows and need to do the same thing Windows users must do to protect yourself.

At this point, I still regard anti-virus for the Mac to be a hit on your system performance for negligible security benefit — it does catch Windows viruses in incoming email, but the only way those will be dangerous is if you forward those messages to a Windows user. The best strategy at this point would be to continue to use common sense — don’t open any programs or attachments that were sent to you via email unexpectedly, especially if they were from unknown users. And never ever type in your system password when you weren’t deliberately installing a piece of software or manually altering a setting on your system, because it means something is trying to gain access to your system illegitimately.