I tried all the suggestions posted so far, without result. I uninstalled (well, down-graded) MSIE 6 (to 5) then reinstalled 6; the "in module Unknown" errors got less frequent, but I got a number of new DLL and VBX errors. I was at one point in someone else's account on the same machine and noticed the errors did not occur. This suggested to me that something in my profile, probably in ..\windows\profiles\<me>\USER.DAT (a part of the registry) was "broken". I did various registry checks, comparisions, etc. and didn't find any obvious problems. So, reluctantly, trashed my user account and created a new one. This seems to have solved most of the problems, and reloading the video and sound drivers got rid of a couple more. It's still not what I'd call stable, but it's reasonable for Win98SE and for my usage patterns (I run a wide and changing variety of software, including some 16-bit stuff.) Now I have to recreate all my menues, preferences, and so on...
No malware of any sort has been discovered by AVG-Free or SpyBot-S&D, or any of my own spelunking through system files and the registry.
Just in case it helps anyone else, I've come to suspect that installing VB4 may have overwritten some files and registry entries with older, incorrect ones. Just a suspicion at this point, however. And possibly not the only problem, anyway.
Thanks to all who responded and good luck to those still struggling with this...
gtippery