I have run for many years without any Anti-Virus software, and I have never been infected because I have Surfin-Guard by Finjan Software.
Surfin-Guard is a passive type of anti-virus that does not require virus signiture files to catch possible viruses. Anything entering your system via email, email attachments or downloaded off the Internet are monitored for any executable files. If that executable tries to do any of the things that viruses must do to reproduce, modify files, access your address book, send emai, modify your registry, etc it is blocked.
In my opinion, it is the only truely effective Anti-Virus that can catch an stop a virus, even if you were the first person in the world to get it.
Of course, if you frequently get files on CD, Diskette, or any other portable file media, you will need a regular anti virus. That is also true if you pass files around on a LAN and all PC's are vulnerable.
...Rich