What is the correct way to manually uninstall AVG from a machine ?
Best practice is to not uninstall using Add/Remove from the Windows Control Panel. This method does not fully remove AVG from the machine, it will still keep remnants of the previous installation which can cause potential future install problems.
The correct way to remove AVG is by using the removal tool found on the link:
Download the link that says "AVG Remover" and run it on the machine.