asdl sharing

I have a ASDL broadband connection which connects to PC with USB. I want to use an old PC as a gateway/firewall (ie broadband connection to old PC then old PC to ethernet switch , and 5 other pcs connected to network.

I was under the impression that I needed two network cards to achieve this, (I assume that broadband connection goes to one and the other connects to the switch box)

a) Assuming that is correct, how can I do this with the USB connectiion

b) if not, what do I do?

I cannot buy any more equiptment at present, so please don’t tell me to buy a ASDL router!!!!!