The cable is a usb-to-serial cable. Windows sees that as a new serial port that has bin connected to the computer, so to download the dives from your computer you actually choose the approperiate COM port.
Which one is the correct then? You could find that out by using the device manager, or just check which ones shows up when the usb-part of the cable isn't plugged in, and which one gets added when you plug in the cable.
//Anton