Click start, programs, right click on my computer and select properties. Select the hardware tab and click on device manager. Then select printer ports and com ports. In the drop down tab your usb lead will be listed, right click on it and select properties. Go to port settings and select com port 1 and click ok. Hope this helps.
I expect you must have some reason for using a USB to RS232 cable. I would just pop down to the local Maplins store and pickup a Null modem cable. On fleabay they are about £2, for that sort of cost its not worth wasting time and energy. I have noticed a few people having trouble with the USB to RS232 cable.