If this is not a permanent state, then “something” is giving you problems.

The “best way to uninstall Avast if normal things are failing” is to use the Clean Uninstall Utility.

Download first the latest stable version of Avast from the official site.

Then download the Clean Uninstall Utility located at http://www.avast.com/uninstall-utility.

Reboot into Windows Safe Mode and run the Clean Uninstall Utility under Windows Safe Mode.

Reboot into Normal mode and use the latest stable version you just downloaded to install Avast. Please REBOOT after that.

I still think that you should double check other security tools too (and first). As I said, “something” is interfering, after some update or alike, that causes this problem to “come and go”.