I've downloaded the new emacs-windows-fullbin.tar.gz (?) from
ftp.gnu.org using FileZilla and now everything works fine, meaning
German Umlaute are recognized to be usual latin-1 characters. I
guess some corrupt char tables were responsible for the bad
behaviour. This might have occured after transferring the
whole emacs-dir from one computer to another via ftp.
Currently I try to incorporate aspell into and the first trials
look good. In addition it is praised to be much better as ispell,
especially for German language.
Special thanks to Eli and Kai for their helping posts.