When you connect the serial cable of your AIS receiver to a USB port (with a Serial-to-USB-Converter), the Windows Operating System will create a virtual COM Port. The number of this COM Port is needed for the configuration of several AIS-related programs.
Follow the instructions to get your COM port number:
Open Device Manager (Start → Control Panel → Hardware and Sound → Device Manager)
Look in the Device Manager list, open the category "Ports", and find the matching COM Port.
Take the number in the bracket behind the port description.