Although I've never seen that error, it looks possibly like the add-in might still be running, or an Office application has started and has loaded the add-in. Have you restarted since uninstalling the add-in?
One possibility that springs to mind is that Office can preload certain parts of the suite upon startup (I think it's called Office Fast Start or something similar..?). That might well also try to load the add-in, which would lock that registry key. Try stopping that process from loading and try the add-in installer again.
Thanks,
Mike