A few years back, I heard about a program called Trojan Remover that was written by a guy who doesn't work for AOL but who ended up becoming the "virus/worm guru" on their virus message boards. He just ended up being the one who answered everyone's questions all the time.
I downloaded his program for 30 days and decided to pay the $25 which was a one-time fee (not annual like with Norton and McAfee) - it's now $29.95, but still cheap by comparison. This thing has caught trojans that Norton didn't on more than one occasion AND FIXED THEM.
The guy who created the program answers all his own email personally -usually within 24 hours. We have bought new computers since buying his program and he has given us new key codes each time with no hassle at all. Great guy. He also updates the virus definitions almost daily, too.
The program does a scan of system files when you boot up but you have to do a manual scan of your computer yourself - there's no clock to set it for a weekly automatic scan and you have to check for updates yourself, too. (When you run a full system scan, you have to disable Norton -- so I just unplug my cable modem while Norton is disabled since the firewall and antivirus seems intertwined).
Anyway, I have no connection to this guy or this company whatsoever, but I've turned a lot of people on to this program.
You can download and use it for free for 30 days here:
http://www.simplysup.com/EDIT: The guy's name is Nigel Thomas.