Neil,
Thanks for the great tool!
Finally, with the tool, I've managed to identify the source of the problem.
Apparently during the reboot, (with the scale still switched on), XP added the scale but wrongly identified it as a Microsoft Ballpoint Mouse device, hence it was hogging COM1.
With the Device monitor, The application was actually closing the port correctly.
Problem now is how do I turn off plug and play detection of devices connected to COM1?
Thanks
Yau