OK.
I tried Avast Repair but that didn't help, I still get a failed installation warning of "avast! Firewall NDIS Filter Miniport".
Next I did an Avast full scan which found 5 instances of "NSIS:Adware-OH, when I applied the Avast fix to them they were sent to the Chest. After this was done I still got the warning.
Lastly I did an Avast boot scan which apparently didn't find anything, Avast didn't mention any problems. After this was done I still got the warning.
I'm quite sure, when I installed Avast, it was the default install. I'm giving some thought to taking the computer to a tech store where I know they uninstall Avast when they check out a computer. Then I would reinstall the custom version, maybe that would solve the problem.