For the love of christ, there are no viruses for Mac OS X. There are some trojans, and there is potential for something to harm your system every time you are asked to enter your admin password. If you become aware of this fact and react with some common sense during these situations, like, "oh, am I installing something that I trust or is it something nobody has ever heard of?" then you have nothing to worry about.
However, if you are on a windows network and don't want to pass viruses on to windows machines through shared files and whatnot, then you can use those antivirus softwares to scan infected files.
That's it. There is no other need for you to use it. Play by the rules, be smart, and you won't get burned. Unlike Windows, where even if you play by the rules, you can still get burned. It's harder now, though. Which is a good thing.