Thanks very much, everyone. I used the opt-out choice when first installing Avast, but I didn’t see it this time, going through the program itself. (Not seeing any such screen, I thought it would remember my original choice.) I was watching it carefully during the update process and there was no opt-out screen; but yes, I did get called away during the install, after the reboot; so by Murphy’s law, it had to happen then…

I used the word ‘force’ appropriately; I almost had to shut down my computer to get it to stop. Finally I used the Esc key, and that worked.