Here's the skinny.
First, I rarely ever post on this site, but I am an hourly leecher.
I just want to get the record straight once and for all. The are no-none-zip-zero viruses for OS X. They're not impossible to write, but the multiuser nature of UNIX makes it difficult to write. This is why trojans exist; they are like viruses, but you need to provide the authentication. These are extremely rare, however, and any OS X antivirus software you install wouldn't be able to detect the virus in the first place, because it wouldn't know what to look for.
Windows, on the other hand, is loaded with tens of thousands. So if you're worried about your Mac being a carrier, then you might want an antivirus, but I wouldn't recommend installing one for OS X. Just install a Windows one on your Windows machine.
I would still never suggest paying for antivirus software, because the companies charging for them are the ones making many of the viruses. Google it, you'll see what I mean. So go open source when choosing.
That's my piece.