Guy(ON4AOI)
We have a new version of the PC client that might fix the problems you see with PTT on XP. There was an issue that didn't show up every time, and not on all computers, not even of same type.
As for the virtual com ports not being installed correctly it seems strange. We are relying on Windows' installer functions to assign a valid and free number for the com port. That is, we don't have control over which port numbers get assigned. Though somehow it obviously fails in your case.
I have scanned the registry and found that the com port numbers are stored under keys like this:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\Root\PORTS\0000\Device Parameters]
in a parameter called "PortName". If you have more than one port there should be one:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Enum\Root\PORTS\XXXX\Device Parameters]
per port. (XXXX=0000, 0001 etc)
Changing the "PortName" to something free might solve your problem temporarily. (Note that the port will still have the same name in Device Manager) Be also warned that messing with the registry might damaged your OS