Hi meilingwall,
I had a similar issue on my laptop and this is what solved it for me:
Open Avast interface and go to settings,
Go to Troubleshooting Tab and uncheck the Enable Avast self defense module,
Now start CCleaner, scan for issues and remove the registry entry,
Go back to Avast and turn the self defense module on again.
Next time you start your PC check with CCleaner and the registry entry should be gone for good.
Let me know if this helped you.