Thank you for your help, but I?ve solved my problem.
Nearly at the same time I?ve cleaned the registry, a patch from Microsoft was installed automatically.
This patch and the monitoring functionality of "free download manager" caused the problem in Explorer.
This does not work. I?ve tried to append a HJT-log from my system, but I was not allowed to do it
(Error Upload failed. You are not permitted to upload this type of file).