This didn’t work.

I restored the default registry from the windows\repair folder, copying into windows\config and renaming the files correctly.

Same problem.

Which leads me to believe that installing a fresh copy of Windows will have no effect because a bootlog shows:

Loaded driver aswVmm.sys
Loaded driver aswRvrt.sys

Loaded driver ??\C:\WINDOWS\system32\drivers\aswFW.sys

In other words, Avast has so corrupted the drive, that I will have to reformat the thing, and that might not even fix it.