In case this helps anyone else here:
Until a few moments ago the following situation existed:
My Avast Internet Security:
1. was disabled. I got a message saying UI Failed to Load
2. Did not allow me to repair it: I got the message Setup is already running
3. Did not allow me to uninstall it: ditto
4. All 6 shields were disabled
5. I had rebooted several times, and also waited nearly an hour to give the 'Setup is already running' message the benefit of the doubt.
In desperation I shut down the computer and rebooted it despite no information messages from Avast, except 'Setup is already running'. And now, it all is working.
I have to say, in my opinion Avast updates are the least user-friendly ones I've ever seen. No real information is provided, and it's unclear from looking at the screen whether anything is happening at all.