I'm having the same problem, I think. I have a Toshiba Portege 7020CT that I've upgraded to XP Pro. It only-sometimes refuses to hibernate, saying that the COM1: port is preventing it. I have to uninstall both the modem and the port and reboot to get it working again for a few hibernations before it refuses again ..... and then I repeat the process. Like you, I'm getting very frustrated.
Rick