Hi,
Did you make sure to click "Apply" after you clicked "Delete" and did you restart your PC?
Could you please follow these steps as below:
Open up "System Configuration" > type "msconfig" from Run or Cortana option. Then go the "Boot" tab.
Avast Clear Uninstall Utility or similar will be listed, click it and select the delete option.
Then click "Apply" and "Ok"
Now Restart your PC.
If the issue persists, please clarify the steps you take and provide screenshots of any error messages you receive.