What is 'avast FA' ?
Where are you seeing this 'Change' button ?
If it is via the Control Panel > Programs > Avast Antivirus > Uninstall/Change, I don't think it will allow you to do that as it is running and protected.
It is most likely that you will have to uninstall it reboot and reinstall and do a Custom install, that should allow you to change the default location.
That said Avast still has to have elements within the C drive so you would still see some things in the Program Files folder (I think this is another reason you simply can't change the location like this), but the bulk would be in the custom installation folder you choose.
