Using ICS (Internet Connection Sharing) - works for Windows 7

How does ICS work?

First, you need one computer, called the host computer, that is connected to the Internet and that has a separate connection to the other computers on your network. You enable ICS on the Internet connection. The other computers on your network then connect to the host computer, and from there to the Internet through the host computer's shared Internet connection.

http://windows.microsoft.com/en-US/windows-vista/Using-ICS-Internet-Connection-Sharing