Argus,
I just pressed the blue button to have Avast do whatever it want to do with the file to fix it - expecting it to not be able to do anything; but this time when I pressed the button the file was succesfully moved into quarantine.
I have just finished another full system scan including a boot root scan, and the virus is gone!
I want to thank you very much for great guidance and help - without you I would still have been stuck!
You are a great ressource to this community.
All the best,