I need access to my USB modem via the serial port. The drivers
installed successfully and the modem is detected without problem.

But,

I always get "Unable to open COMXX" for alot of random values of XX. I
have tried re-assigning COMM port assigned (via Device
Manager|Modem|_My_Modem_|Advanced Settings| Port number) and rebooted.

I can Query the modem successfully (via Device
Manager|Modem|_My_Modem_|Diagnostics|Query).

There are no conflicting entries for the modem pid in the c:\winnt\inf
folder.

I have cleaned the registry (a number of times) of the pid of the USB
modem and also scanned for suspicious entries for COMXX.

Sysinternals "Process Explorer" shows that the only processes with
'serial' handles open are hypertrm.exe and System (using
HKLM\SYSTEM\ControlSet001\Services\Serial).

I'm really struggling with this and am at a loss as to what to try
next. Any help much appreciated.

tia
Ben