Thank you for your reply. It's leaves me with a number of unanswered questions.
Why would two installations of W7, on the same model Intel MB, installed from the same DVD (different product keys), with the same memory and drives, choose different COM port assignments?
If the COM port numbers are randomly assigned by Windows why would motherboard documentation specify the COM number for the ports?
Why, in many years of working with PC hardware, have I never experienced COM port numbers switching when wiping or replacing a disk and reinstalling the OS?
Sorry, I'm not convinced that the hardware or maybe BIOS didn't play a part in this.