avast generally is not a problem when uninstalled, like many other AVs, so don't let a previous problem stop you trying. I would say don't wait, you have a problem that you should resolve, the next program update may not resolve the problem, if as oldman it could be a failed or corrupt install.
First Download the latest version of avast
http://www.avast.com/eng/download-avast-home.html (scroll down to the German language version) and save it to your HDD, somewhere you can find it again. Also download the Uninstall Utility as oldman said before you uninstall the current version.
Now uninstall using add remove programs and reboot.
Run the uninstall utility and reboot.
Install the latest German Language version you downloaded.
Do you install avast in the default location, C:\Program Files\Alwil Software\Avast4 ?
If not try using the default location, or ensure that there are no accented characters in the path/location you choose, like umlauts, etc.