when you install the lead to your computer and install the driver from the CD it will assign a communication port for the device. you can find out which one it is using by going to device manager in windows XP (right click on the my computer icon, left click on properties in the list, this brings up a window. choose the hardware tab and click on device manager) find ports in the list this brings up, click on this and find usb serial port. double click this and find which port it is assigned to. you want it to be set to a port between 1 and 4. you you can change it if required by then clicking