You could have a modem or other device using the port, depends on how XP is set up, the hardware your PC has, etc, etc, etc... I've never used XP myself so I can't help you there... I run Win2K, and my PC is set up to interface with the TEMS units and the FADEC's.
I'd bet its a problem in your PC. All I can say is keep fiddling with it and suddenly it'll work.

As strong said, an email to Gaspar wouldn't hurt either.
Good luck,
KW