Keep in mind that if you run Windows, either via Bootcamp or as a virtual machine, you do need anti-virus software. Your Windows hard disk partition offers malicious code an opportunity to get onto your hard drive.
The general consensus now is that anti-viral software is not needed. That has been a source of much discussion. Here is an article from Apple. If I'm not mistaken, the story was retracted. But, some caution is always a good idea. Keep informed about what is happening in the world of computer security. Keep your version of OS X up to date with the latest security updates released by Apple.
http://www.informationweek.com/news/...leID=212201455