I can't uninstall Avast from my PC.
If you have issues uninstalling Avast, you can use Avast Uninstall Utility available for such cases. Download the application and run it. In a few moments, it will be removed from your computer.
The application already features an uninstaller accessible from the Add or Remove program but in case you can't uninstall it via the usual way, I recommend on using the tool released by Avast for situations like these.