hi,
so it's OK now
I don't know why, but the default rules of the avast "Windows Networking IN/OUT" firewall, which is supposed to block port 445 (TCP) ... were not taken into account.
I created a rule manually and put it at the very top, and since then the network inspector tells me that my device is secure.
Using avast premium, I find it a shame that when I click on "resolve", avast can't automatically create the rule in its own firewall ... It just gives an unclear information.