Hi, Alikhan, Thank you for your reply.
Some of your instructions I had already done, than I followed the other ones, but no way, it never started. Then I searched through directories and files and found there was another directory named Avast with the same usually installed files, as a second installation. I cleaned them up with Avastcleaner to also delete their corresponding registry keys, as I thought it would do. After this I reinstalled, updated and it was fine as before. What a headache!
Thank you for your help and good will.