What is the problem. If the modem is USB it is not connecting via ethernet. If it is an ethernet/USB modem combo and it is currently plugged in to your network card, then do the following: 1) unplug the ethernet cable from the modem. 2) plug in the USB cable. 3) run the setup utility. It's as simple as that.
If you have internet service and the modem is connected to your computer via ethernet cable, and your connection is through your network card, you're still connecting through the modem.
Please realize there is a rather large difference in speed and processor utilization between USB and ethernet with a cable-modem, as well. Perhaps you could receive more help if you said why you're trying to do this??
/L.A