duke is right.
I think -
a simple network kit will serve you all needs:
assume you already got a NIC connecting one pc and the cable modem. Put one NIC into each of ur pc's, assign them bogus ip internally like the 192.xx.xx.xx. hook up the two new nic onto the hub. grab a free proxy software from tucows, and install it on the pc connects to the cable modem. set up the proxy might be a little hassle, but no pain no gain...