In current releases of Windows, the comX ports no longer map to the old defined addresses. It is possible that heatwatch is looking for a serial port IE 1 at 3F8h, 2 at 2F8h, 3 at 3E8h, 4 at 2E8h. This could mean it is attempting to take direct control of the serial port hardware. If that is the case, the USB one may never work... If you go in to device manager and show hidden devices, you can delete the other drivers and get the ports to select a different address.
Just a guess at the moment.