Thanks for your reply!
Some of my programs need administrator rights to run, and they are used by other users, that why I have more than one account with those rights.
I have tried to use the same key in all accounts, but it failed: Invalid licence number.
Did you try to logon and install with the Administrator account?
Yes, I have done that, with the same problem.
I have noticed that some Avast tasks is running in all accounts:
ashServ.exe, ashMaiSv.exe and ashWebSv.exe
If they can protect my computer when I use those accounts, it will be OK. I don't need to access the Avast window with all accounts. But I want to be sure of that. Do you know something about that ?