OK, I think I've included everyone who posted to this thread. Here's what I ended up doing:
After trying pretty much everything that had been suggested, both on the thread and on FReepmail, I repartitioned and reinstalled WinXP. Then I updated to SP2.
The only casualty so far seems to be the Epson Perfection 1650 scanner. The software just simply will not install. It keeps getting hung up at 78%. I don't see any physical problems with the CD itself, so I don't know what the problem is.
There is one software problem, and I think that is fixable. Calendarscope refuses to accept my registration number. I've emailed tech support, but they're in Russia, so I'm hoping they can fix it. (After I get it back, I'll spend the next few months copying appointments and tasks to Mozilla's free calendar app, which is called Sunbird 0.2 right now. It's supposed to be ready by the end of the year, and I'm using it now.)
Anyway, I had to repartition and reinstall because, in the final analysis, the system wouldn't let me install anything, a WinXP Repair install didn't correct any of the basic problems, and because it wouldn't Save Settings, I could never make any solutions stick.
Thank you, every one who posted here, even those who said they couldn't help, but whose posts bumped the thread to the top where others could see it.
Thank you.
I have an Epson Perfection 2400 scanner and it loaded fine.
Load the software before you plug the scanner in and see if that helps.