EDIT: Apparently I took way too long writing this...
Please, please, don't go download and install an anti-virus program! There are many reasons, but the most significant is that none of the anti-virus programs in existence can catch a Mac virus, because there are none to catch (most don't even detect the few trojans that are out there). Because of this fact, you're looking at wasting a significant percentage of your computer's resources (mostly RAM and processor power) for zero benefit, and arguably placing yourself at more risk because of the false sense of security it brings.
All you need to do to protect yourself is be aware of exactly what is asking for your password and why. If you don't type in your password for things you didn't intend to install and that you know where they came from and why, then you'll never have any problems with malware of any type on the Mac.
If you are sharing files with a PC often and feel that you absolutely must have something to scan for Windows viruses, ClamXav is one of the least intrusive and more effective tools for that. I wouldn't recommend letting it scan in the background, just run it on files you plan to transfer. Definitely don't use iAntiVirus, it actually is almost malware itself. GGJStudios will probably show up and tell you more about that.
jW