You need a CAT 5 Cross Over cable. The CATegory 6 one will do but i suspect will be unneccessarily expensive. Connect the Modem to one PC via the USB and then run the CAT 5 Crossover Cable from one PC network port to the other PC network port.
You then need to set up Internet Connection Sharing (ICS) - click here
Note the Host PC with the modem must be on for the client PC to have an internet connection.