from dsl to cable - "a network cable is unplugged"

I used to have dsl but can no longer get it in my area. I have switched to comcast broadband with the motorola surfboard 5100 but the computer won't recognize the modem via the ethernet cable. I can get connected via usb slot. Eventhough I no longer get dsl service, if iI plug the dsl modem into the ethernet I get a connection.  There is no Verizon dsl software left on my computer and when I load the comcast intall program it won't go any further until the network cable is plugged in. Any ideas? Oh, I have to use the Ethernet connection and not the usb. Thanks