Would you run an AV on Linux or FreeBSD or Solaris, etc? Of course not, so why run one for NT which has at least the security capabilities of those other systems? The only systems that benefit from AVs are those with poor architecture that allows random processes unmitigated access, like the Windows (SUE)line Single User Edition. Restrict administrative accounts from running untrusted applications and isolate/restrict standard users in a manner that prevents the virus from being able to propigate.This has the advantage of being immune to new viruses and trojans while requiring no upgrades. Besides, if you have your system setup in a manner that makes virus propigation not possible, why waste the time scanning?
Secondly, I would hope you are all aware that ALL firewalls are software, some just run on very limited operating systems rather than general purpose opersting systems and on specialized hardware rather than general hardware. Firewalls should be divided by type or generation, since this actually allows for a sane comparison. Lastly,before we get some replies, are you to take the word of the masses here? Something about the "least common denominator" should ring true.