there should be some drivers that you can install with the USB-serial thing. once its installed restart ur pc. when u plug it in to the usb device, it should automatically assign itself as a virtual com port.
go to control panel and device manager and it should be there under "com & lpt" or something.
if it is not, click on "universal serial bus controllers", and find which one is the device which u have inserted.
right click on it, click properties, then under one of the tabs it says "load vpn" or something . make sure that is ticked.
confirm all changes, then in device manager do "scan for new hardware".. if nothing comes up unplug the device and plug it in again, it should then work and have created a "com port & lpt" heading in device manager, which u can click on to see which virtual com port it has been assigned to.
if this doesnt work im not sure what it can be
hope it helps