you will have to make some changes to the way your network is setup - ie tell internet connection sharing to share the ethernet connection instead of usb (i am assuming that you have an ethernet card available and installed in the main pc)
process is simple... shutdown pc and unplug power to the cable modem, connect the ethernet cable instead, when sync and rdy on the modem (or cable if it is a terayon/motorolla modem) are solid turn the pc back on... all done
