how can I get such a uninstall-question in my uninstaller:

This question should only appear if there are 'user-created files' (which not includes in the default-installation) in the $INSTDIR.
3 posts

hope this helps
MessageBox MB_YESNO "Would you like to delete all Files in the Folder?" IDNO done
delete:
RMDir /r "$INSTDIR/Folder"
done: