I had no trouble with the retail copy of Norton Antivirus 9. Had to test it out before recommending it to a company I was dealing with that was passing on infected files to clients PCs. Of course, the files had no impact on their Mac systems, but it made dealing with other people a pain in the neck. So, like others have said, if you don't deal with PC users, you don't need to worry about it too much, though you never know when someone will write some nasty virus for Mac OS X just to prove a point. The problem with Virex I found is that 1) it only comes with a .Mac account, 2) you have to manually scan stuff. VirusBarrier wouldn't let me connect to my PC shares for some reason. Sophos is cool, but is geared towards large corporations. Norton (with the updated Panther UnInstaller) worked very well. Any program that you have running in the background all the time will use memory, so if you don't have a lot of memory, you will probably notice it.