While I'm in the mood for posting solutions to technical problems, here's another. I have a OBD interface cable for my VW Bora so that I can run a piece of software called VCDS (sometimes known as VAG-COM) and read the error codes from my car's ECU. However, when I plug in the cable (it's the USB version), it is usually assigned to COM7. Unfortunately, the VCDS programme only supports COM1-4.
Weirdly, the solution for me is to plug the cable in and run VCDS settings. If I choose COM3 then it successfully detects a COM port - I think it's used by the built in model. I save the settings, and then bring up the Device Manager and change the port used by the interface cable to COM3. It says the port is already in use, but lets you change it anyway. Then I just use the VCDS software and it works. This seems completely wrong, and to be honest, I'm sure there's probably a better way of doing it than this, but at least it works.