Case 2) Adding wireless to an existing Ethernet network - Internet sharing via NAT software.
You can also use a software method, such as the Internet Connection Sharing (ICS) feature in Windows 98SE, Me, XP, or 2000 to share your Internet connection. In fact, this will be the only method you can use to share your Internet connection if your BSP supplies you with a cable or DSL modem that connects either via USB or plugs into a PCI slot inside your CPU's cabinet. See Figure 2 for the connection details.
Figure 2 - Network with Internet sharing CPU and Access Point
Ositis' WinProxy is an alternative to using ICS, and it provides a number of features that ICS either doesn't have or are hard to use. Ositis has a little tutorial that you can check out for more info.