It is my opinion (and many others, I'm sure) that running anti-virus software for Mac OS X is rather pointless.
Of course, if you're going to run Windows on the Mac, then yes, you should install anti-virus on Windows, as with any Windows installation. (in which case I would recommend AVG Free Edition)
People come to me with their pen drives and ask me to clear all the viruses. I just do a spotlight search for .exe and delete them. They don't affect OS X...
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