There aren't many (compared to PC) viruses for the MAC, but they are out there, from OSX/Leap-A to Newton and iBotnet and maybe 60(??) others, but you only need to get one to have a bad hair day. And from what I hear, the MAC ones are nasty.
I would only run a computer (any kind) without AV software if 3 things can be 100% assured.
1. No new software is to be installed or data copied from floppy/cd/external drive/memory card and no provision for such access is installed.
2. No internet connection and no provision for one (no RJ45, USB slot, 1394, wireless or RJ11).
3. Computer is physically secured. No unauthorized users.
I clean viruses (among other things) for a living in aerospace so I am not without experience here and my take is, if it can receive data from another computer, in any form, it needs AV software.
For the PC, I agree with TsPoet; Avast is quite impressive for 'Free for home use' software, and I wish we used it at work. I was not impressed with AVG when I tried it years ago, and Mcafee is a bit of a resource hog and is hard to remove.
Tzvia- rollin' slow...
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