Fixed it!
Your questions led me to read the installation procedure for the modem more closely. The phrase "select an empty PCI slot" rang a bell. In the list of hardware devices I had noticed that there was a PCI Controller without a driver. I installed the modem driver to this, it immediately detected the modem and installed it on Com 4. Oddly enough Com 4 doesn't appear on the list of ports, however the modem properties detail it as being installed on Com 4.
Thanks fobotsco and Naples Air Center, Inc. for your help.
