I thank you all; I think the problem's solved.
Igor's certainty that the problem was caused by "information returned by the operating system itself", and a couple of hours sleep, got me to thinking, finally.
I was running the Avast installation program as Administrator, and I wasn't running the Avast installation program in compatibility mode directly, but I was launching it from ACDSee Classic (circa 1998) an image viewer/file manager that WAS running in Windows 98 compatibility mode. The thought that compatibility mode was inheritable would never of entered my mind without help.
I've been sorting a bunch of image files using ACDSee because it supports 4DOS descript.ion files and running ACDSee in compatibility mode because animated GIFs cause ACDSee to lock up under any Windows version after Windows 98SE.
All of the programs I mentioned above that would not install, including Avast, will at least start to install when launched from Windows Explorer. I'll let you know if Avast will fully install when launched from explorer, but first I'm gonna get some sleep.
Now I'm wondering if this compatibility mode inheritability has or will cause problems with any of the programs I've previously installed by launching them from ACDSee.
Thanks again for all your help.
Gerald