All of the computers at my college use
sophos, which cost money although they've got a
free version for macs.
I just throw MSIE on my computers. I don't really get any viruses in any case. In my experience, keeping browser plugins up to date, turning off scripts in Adobe Reader or using a reader that doesn't support them, and turning of autorun (especially if you use flash drives) is as important, if not more important, than having a good antivirus program.
I also load sites that I don't trust in a sandboxed browser in a virtual machine, or use a text-based browser to view it. That's probably overkill, if you turn off images/flash/javascript/active x/all plugins in a normal browser it will have pretty much the same effect. In any case, the simple act of not going to sites you don't trust will protect you from a surprising amount of bad stuff.