How to Enable ICS on a Home or Small Office Network Connection in Windows XP
 

I found this in a few different places around the 'net - hopefully it's helpful:

How to Enable Internet Connection Sharing on a Home or Small Office Network Connection in Windows XP

This article describes how to share one Internet connection on your home network or your small-office network.

Through the Internet Connection Sharing (ICS) feature for network and dial-up connections, you can use Windows XP to connect your home network or your small-office network to the Internet. For example, you may have a home network in which a Windows XP-based computer connects to the Internet by using a dial-up connection. If you enable ICS on the computer that uses the dial-up connection, you can provide network address translation, addressing, and name resolution services for all of the computers on your network.

Note that, for Internet Connection Sharing to be enabled, the Windows XP-based computer must have two network adapters, one for the home or small-office network, and one for the Internet connection.

Setup Procedures
To enable ICS on a network connection:

• In Control Panel, double-click Network Connections.
• Click the local area network (LAN) connection or the dial-up networking connection that you want to share (that is, the one that connects to the Internet), and then, under Network Tasks, click Change settings of this connection.
  
• On the Advanced tab, click to select the Allow other network users to connect through this computer's Internet connection check box.
• If this is a dial-up networking connection, and you want the connection to be automatically dialed when another computer on your network attempts to connect to the Internet, click to select the Establish a dial-up connection whenever a computer on my network attempts to access the Internet check box.
  
• If you want to allow other network users to enable or disable the shared Internet connection, click to select the Allow other network users to control or disable the shared Internet connection check box.
  
• Under Internet Connection Sharing, in Home networking connection, select the connection that connects the computer that is sharing its Internet connection to the other computers on your network.

Note that to enable ICS in Windows XP, you must have administrative rights.

IMPORTANT: When you enable ICS, the network adapter that is connected to the home or small-office network receives a new static IP address of 192.168.0.1, with a subnet mask of 255.255.255.0. Existing TCP/IP connections on the network may be lost and must be reestablished.

Configuration and Usage Issues
The ICS feature is intended for use in a small office or in a home office in which the network configuration and the Internet connection are managed by the Windows XP-based computer on which the shared connection resides. It is assumed that, on this network, this computer is the only Internet connection, this computer is the only gateway to the Internet, and this computer sets up all internal network addresses. All hosts on the network except the ICS host are expected to be configured to obtain IP address and DNS configuration automatically.

You cannot modify the default network configuration after you enable ICS. This includes changing the range of private IP addresses that are handed out (DHCP allocator), enabling or disabling DNS, and configuring a range of public IP addresses.

If your home office users need to gain access to a corporate network that is connected to the Internet by a tunnel server, the users need to create a virtual private network (VPN) connection to tunnel from the computer on the ICS network to the corporate tunnel server on the Internet. The VPN connection is authenticated and secure, and creating the tunneled connection allocates proper IP addresses, DNS server addresses, and WINS server addresses for the corporate network.

You may need to configure programs and services to work properly across the Internet. For example, if users on your home network want to play a game with other users on the Internet, the game must be configured on the connection in which ICS is enabled. Services that you provide must be configured so that Internet users can gain access to them.