1. Use the avast uninstall utility in and delete the hidden Avast folder in the hidden Program Data Folder.
  2. Then download your version (free/pro/etc) from here:
    http://forum.avadas.de/threads/5860-Wo-ist-der-Link-zur-letzten-8er-avast!-Version?p=49858&viewfull=1#post49858
  3. Make sure you don’t get updated higher or accidentally click on the GUI to automatically resolve problems after you’ve turned of file shields, because it updates you to a version over 8.01500 which are automatically updated to 2014 for Windows 8/8.1 users.

(Though I have to admit that I didn’t test it through, because I’ve updated it by mistake to 8.0150X and have already completely uninstalled it before it could update itself.)