Thanks.
But unfortunately i ran into a new problem.
After struggling for a while, I finally got to uninstall the previous version of Avast in Windows Safe Mode.
The problem now is that I can't run any of the 12 executable installers in the link you provided (4 Beta online installer , 4 Beta offline installer or the 4 regular installers).
The message is the same as before: "Error: Windows cannot access the specified device, path, or file. You may not have the appropriate permission to access the item"
I checked this message in the Microsoft site and I don't fit in any of the possibilities listed (no other antivirus installed, file not blocked, other executables in the same folder run fine, Properties>Security>Permission are all checked for all users)
I also tried an install from the Avast site, as if I were a brand new user, downloaded the install file to a different directory and still have the same issue.
Any hints?