There is an application that is probing serial ports (or USB devices) when a device change notification is sent by the operating system. The usual culprit is a PDA / mobile phone application.
As far as I know nothing in the Windows XP operating system behaves that way.
Well OK then! I'm so glad I asked here before wasting too much time trying to figure this out. (I somehow doubt I would have, actually... figured it out, that is.)
Thank you kindly for your wisdom and assistance, sir.