My money's still on Norton Anti-virus. I use Norton Systemworks 2001, and am very happy with its performance - even under WinME (!)
The single best feature for me is that when emails are downloaded by Eudora, they go through NAV first. Any infected attachments, embedded trojans etc. are picked up and you're given options to delete, clean or quarantine the file(s) infected.
This great and unobtrusive feature has netted a few nasty-looking viruses and saved no end of grief. The updating process for virus definitions is also quick, effortless and free.
It also keeps an "eye" open for web pages with harmful java scripts and other such technical stuff, but so far so good on that score.
Good luck.