How could I correct that the way I could open html files etc. with Firefox instead of with VLC: http://i.imgur.com/RRj3XQN.png

And I have installed "PDF Architect 2". Before automatically the PDF files have been opened with PDF-XChange Viewer, but now they are opened with "PDF Architect 2". How could I automatically let them open with PDF-XChange Viewer again?

This is because you did it outside KeyFileAssoc tool integrated with Liberkey Menu.

You would be able to remove those associations but please close Liberkey Menu (or at least KeyFileAssoc) before performing any registry modification:
http://www.howtogeek.com/171932/how-do- ... windows-7/

OK, many thanks for the link, Dji.

I will try it.

Many thanks again.