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.