Problem solved!! Although I don't know what caused it in the first place.
I found a hard copy of my config.sys and autoexec.bat files buried deep in my desk, typed them in with Edit, and presto, back in working order.
For TT4U: I had originally entered MS-DOS mode simply from the shutdown procedure: Restart in MS-DOS mode.
The application that I wanted to run (but never did) was one that had been on my system for years. It was NOT a new thing, so its installation could not have changed any current settings. It is an old DOS-based virus scanner, by the F-Protect people. Booting out to MS-DOS mode and running the scanner is something I have done a lot. On this occasion, just as I was about to run the scanner I realized that I had not downloaded any new virus definitions, so I wanted to return to Windows mode to do so, come back to DOS, and then run the scanner. I never made it out of DOS.
For Wolluf: typing WIN and EXIT at the command prompt produced the same result: I rebooted back into DOS mode. And oddly enough, msdos.sys had BootGUI=1. Weird!
I see that I wrote, "After entering DOS mode I changed my mind and decided to try it in a DOS window." This is indeed true, but I must have had a brain fart at that moment! After much reflexion I remember now the reason why I had developed the habit of running the scanner in DOS to begin with: the distributors of the program caution that running in a DOS window ran the risk of producing false negative results from the newer generation of viruses that knew how to hide while in Windows. Indeed, this old warhorse has found one virus and two pieces of malware for me while run out in native DOS, all of which were missed by AVG and SpyBot.
Thank you everyone. The question is now academic. The problem has not recurred, and I have booted out to DOS several times now with no anomalies. But it would be nice to know what can cause this!! I had not installed any new software to my system for several weeks, so there were no new settings that may have caused conflicts.
Best,
Mark
(marquitico)