At last a question that I really do know the answer to, that is if you are running XP. It may well work for other versions of windows.
Plug your adapter into the USB port. After the Bing-Bong select the following:-
Control Panel > System > Hardware > Device Manager > Ports
You will see the USB to Serial port listed with the assigned Com port in brackets.
It will probably be COM 5.
Set that as the port to read, and off you go.
"A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools" Douglas Adams