I don't use any virus protection myself since they're really aren't any credible viruses out the for OSX. Either way, stay away from ANY AND ALL Norton products for Mac! They will cause you nothing but problems.
With that out of the way, check out ClamXav here:
http://versiontracker.com/dyn/moreinfo/macosx/24449
It's free and people speak very highly about it. Hope that helps.