Is the Virgin Media broadband via cable? If so, that's what I have got.
With the cable modem you have the choice of connecting to your first pc by usb or by ethernet, hence the 2 sockets. (NTL used to recommend using the ethernet option)
To connect a second pc to share the connection you need a cable router. Both pc's are attached to the router via ethernet and the router is connected in turn to the modem.