We recently had to obtain a new registration key as AVast was starting to expire. My daughter didn’t take care of it through her email and Avast expired. My wife reapplied for Avast or reregistered and somehow got confused and downloaded AVG too. I uninstalled AVG. Got the new registration key from her for Avast and completed the process. And now Avast won’t open, but it’s there. Can anyone help me?
Ahem, after rebooting, it’s working. 
It is probably worth using a registry checker to see if there are any registry entries left behind after uninstalling AVG.
AVG lefts tons of Registry keys behind, lots of Legacy keys 
Some of them you can only remove manually, one by one… 