I plugged and unplugged the offending Nano while watching what changed in the list of Coms Ports in Device Manager. That way I saw that it was actually Port 13 and not19 that Air Manager was looking for. I went in to the PC's Registry and renamed it to something sensible - "Annunciators Nano...
I have had a particular Arduino Nano working in my PC for about 13 months. Now it doesn't. When I connect, or unplug it, I hear the Windows "device connected/disconnected" sound. However, it does not show up in Device Manager's Com Ports list. (It used to be Com19) Any ideas how to get it back?