i typed ICS into xp help and support and came back with this.
To enable Internet Connection Sharing on a network connection
Open Network Connections.
Click the dial-up, local area network, PPPoE, or VPN connection you want to share, and then, under Network Tasks, click Change settings of this connection.
On the Advanced tab, select the Allow other network users to connect through this computer's Internet connection check box.
If you want this connection to dial automatically when another computer on your home or small office network attempts to access external resources, select the Establish a dial-up connection whenever a computer on my network attempts to access the Internet check box.
If you want other network users to enable or disable the shared Internet connection, 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 any adapter that connects the computer sharing its Internet connection to the other computers on your network.
You should not use this feature in a network with other domain controllers, DNS servers, gateways, DHCP servers, or systems configured for static IP.
When you enable Internet Connection Sharing, the network adapter connected to the home or small office network is given a new static IP address configuration. Existing TCP/IP connections on the Internet Connection Sharing computer are lost and need to be reestablished. For example, if Internet Explorer is connecting to a Web site when Internet Connection Sharing is enabled, refresh the browser to reestablish the connection.
To use the Internet Connection Sharing feature, users on your home or small office network should configure TCP/IP on their local area connection to obtain an IP address automatically. Home or small office network users must also configure Internet options for Internet Connection Sharing.
To open Network Connections, click Start, click Control Panel, and then double-click Network Connections.
You must be logged on as an administrator or a member of the Administrators group in order to complete this procedure. If your computer is connected to a network, network policy settings may also prevent you from completing this procedure.
Internet Connection Sharing and Internet Connection Firewall cannot be enabled on incoming connections.
For information about the protocols, services, interfaces, and routes that are automatically configured, click Related Topics.
To enable Internet Connection Sharing (ICS) Discovery and Control on Windows 98, Windows 98 Second Edition, and Windows Millennium Edition computers, run the Network Setup Wizard from the CD or floppy disk on these computers. For more information about the Network Setup Wizard, click Related Topics.
For ICS Discovery and Control to work on Windows 98, Windows 98 Second Edition, and Windows Millennium Edition computers, Internet Explorer version 5.0 or later must be installed.
The Home networking connection is only present when two or more network adapters are installed on the computer.
Internet Connection Sharing, Internet Connection Firewall, Discovery and Control, and Network Bridge are not available on Windows XP 64-Bit Edition.
Anyway, ICS in short:
1. Enable ICS on the cable modem connection on your "host" PC.
2. The other NIC will get 192.168.0.1 as IP.
3. Enable DHCP (automatic IP) on the other computer.
4. Make sure it gets an IP (192.168.0.xxx).