How to fully uninstall my Avast after trial period ends?
You can use Avast Un-installation utility available from the official website or you can try using Revo Uninstaller. Even if it can't uninstall the application normally, it will force it to uninstall by wiping out all traces of its presence from the system.
After that, try running AdwCleaner to make sure that there are no leftover rogue registry keys or toolbars in your system.