usb (prolific) to rs232 serial cat control

My experience of using wspr to control ft897 through usb port using Prolific usb->9 pin rs232 (XP and wspr 2.21 v 2286):

Doesn't work unless you force the pc to assign com1 (perhaps up to com3 but not com4 and above)

ctrl panel->system->hardware->device manager->ports->port settings->advanced and set the com port to com1.

Not a fluke 'cos same on two pc's