I use InocualteIT PE. It's free. I'm not 100% sure if it's the best, but for free, you can't complain. It offers realtime protection, and you can set it up to check for updates on the web automatically (works good on like a cable or LAN. I haven't tried it on a dial up)

We run a version on our Exchange server(you have to purchase a license for this version) at the office, and it does good dectecting the viruses, then making them non executable by renaming the file to *.avb It even notifies the admin by email with a report of viruses when you run a full Exchange scan. I probably gave you more info about the software than you wanted, but it could help someone.

http://antivirus.cai.com

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OS/400...At least it's not Microsoft.