If you have a problem with COM port devices under Windows 7 or later, your system may think it has autodetected it as an old-style RS-232C port mouse! To fix that, regedit here:
hklm\SYSTEM\CurrentControlSet\Services\sermouse
and change the DWORD Start
to 4. This prevents the autodetection of COM-port mice!
Contributed by the excellent Brad Hollingsworth.