Well, solution found after a one day stress...
The use of the Windows task manager has been a key to be able to start "Explorer.be" which set my laptop (only for the session) in a normal way of work.
From this I have used System Mechanic" which found my laptop ready to use....What about this unrelevant diagnostic
Then I came back to Avast and I have seen it was possible to 'undo' requested registery removes.
Doing this, I can start again my laptop in a normal way.
What is the lesson learned? From my side, I know. What about Avast? I would be interested for some additional admin feedback.
That said, I keep my confidence for this tool as there is an undo option that have saved me.
From Belgium
Marc