Hi Ivo, welcome to the forum 
Try a clean install :
Download the Avast Uninstall Utility to your desktop.
Download Avast Internet Security
Disconnect from the internet.
Uninstall Avast Internet Security via control panel.
Run the Uninstall Utility and accept the reboot to safe mode.
Once complete reboot your system.
Reinstall Avast Internet Security.
Greetz, Red.