Did you try my solution to the missing file (see the bottom on my post above)? That’s what I had to do to get it to work.

I even seem to recall an error message along the same lines as you say.

  1. go to your Avast installation directory
  2. go to the “Setup” subdirectory
  3. from Explorer, rt click New->Text Document. Call it “avast.setup”
  4. set permissions for this file in PG (as shown above)
  5. delete the avast.setup file.

Good luck