abrundige -- have you tried uninstalling Avast from the Control Panel? Can you tell us if the uninstall performed successfully?
If you are trying to delete files manually, it's possible you'd need administrator permissions to do this, depending on the file.
We have an uninstall program that's designed to fully remove Avast if for some reason the standard uninstall doesn't work -- you can find that here:
https/www.avast.com/uninstall-utility
We hope this helps!