My mother's Win98 box throws an 'LUCallbackproxy.exe' error on logout or shutdown. As near as I can tell, it's a Norton Antivirus bug with Windows 98 that they're not interested in fixing.
So that rules out NAV. Next I looked at McAfee. Lord, but those Amazon reviews are nasty. The NAV reviews are almost as bad.
PC-Cillin has decent Amazon reviews, in line with less trustworthy CNET editor and user reviews. Looks like the antivirus marketplace for Windows will be Microsoft Live for XP and Vista, and PC-Cillin for everything else.
(Soon she'll inherit my XP box. I'll replace it with a MacBook laptop and boot XP as needed. That XP will run without antivirus software -- it won't be doing much on the net ...