Back in the old days you had to set up Nat on a computer with two Nics if one was connected directly to a cable modem ect.
http://support.zen.co.uk/kb/Knowledgebase/Windows-7-NAT-Setup
How to find your default Gateway/ internet connection.
Click on the start pearl, in the search box type Cmd press enter.
Next in the dos box type: ipconfig/all
You can obtain the gateway address from the first Nic card, that the broadband modem assigned it.