[Walter]

noch eine Lösung:

1. CTRL-Alt-Del to bring up Task Manager.
2. Click File | New Task(run).
3. Type regedit in the Run box and click OK.
4. Browse to the following registry key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution.options

5. Under this key there will be subkeys named explorer.exe and iexplorer.exe. Delete the explorer and iexplorer keys entirely. They should not be listed under the Image File Execution.Options key.
6. Close the Registry Editor.
7. Restart the computer.

funktioniert !