Two parts to this, which I suspect are related:
1. All was okay until I rebooted; for example, the password problems described below did not exist, as I had entered the password in question more than once before the reboot and those entries worked correctly.
2. When I rebooted, the password box was filled, instead of being empty. I removed whatever was in the box, and typed in my password, which was rejected. The password in question, which I use to log on and for two programs, is in the format 111XXX; I normally type in the numbers, then hold down the shift key to enter the upper-case letters. After some consternation, I discovered that my password was accepted IF i typed in the numbers, then engaged the caps lock, and without holding a shift key, entered the upper-case letters.
3 I subsequently found that this odd need to engage the caps lock is also true for the two programs that use the same password as I use to log on to the computer.
4. None of this seems to be caused by a keyboard problem, as I can enter URLs using the shift key for upper-case letters, I can type documents correctly, and so on. MalwareBytes shows nothing, my anti-rootkit program shows nothing, and so on.
5. My automated password insertions work correctly, such as my logging on to this forum, which uses a password with numbers and both lower- and upper-case letters.
Again, I suspect, but have no way of knowing, that the oddity is caused by whatever is filling in the computer log-on box when I reboot. Solution suggestions are solicited.
After posting the original message above, I ran an Avast! boot scan, just to be certain; it found no viruses. When it completed the scan, Windows started, and the log-on box was empty, so I typed in 111XXX, with caps-lock off and using the shift key for the upper-case letters; that worked. I then used the same method successfully with one of the two applications that uses the same password.
The only thing that was different when I encountered the problem was that my DSL line was live, but there was no Internet, a condition I don't recall having encountered before. I don't get ISP failures often, but when they do happen, it is usually the case that both DSL, and, therefore, Internet are down, and I have rebooted in that state and had no problem. Is it within the realm of possibility that having DSL but no Internet was somehow affecting my Windows log-in and the use of that password with applications?
(I'm not going to mark this as solved, awaiting any possible input from those who have an understanding of this situation.)