Un altra bella guida, presa sta volta direttamente dal sito ufficiale di xbox-live (il procedimento è lo stesso, almeno in teoria, anke per GameCube ^__^).

Windows ICS Connection

You need to use a Windows ICS connection if your cable or DSL modem is USB-based, or if it is an internal PCI modem. Otherwise, you should choose a different connection type.

A Windows ICS connection uses Microsoft® Windows® Internet Connection Sharing (ICS). ICS lets your Xbox and your PC use your Internet connection at the same time, but you must connect your Xbox console through your PC, and always turn on your PC before playing games on Xbox Live. Xbox Live is supported through Windows ICS for Windows XP, Windows Me, Windows 2000 Professional, and Windows 98 SE.

Connect with a USB-Based Modem

ICS @ Windows XP

To use ICS to share your Internet connection, the host computer must have one network adapter configured to connect to the internal network, and one network adapter or modem configured to connect to the Internet.

Log on to your PC as Administrator or Owner.

In Windows XP, ICS is automatically loaded during installation.

On the taskbar, click Start, right-click My Network Places, and select Properties.

Depending on what you have installed on your machine, you can connect using either a Local Area Connection or a PPPoE connection.

Local Area Connection

Below the LAN or High-Speed Internet heading, right-click Local Area Connection.

On the shortcut menu, click Properties and then click the Advanced tab.

Below Internet Connection Sharing, click the Allow other network users to connect through this computer's Internet connection check box.

Click OK.
The connection to the Internet is shared to your Xbox console. TOP
PPPoE connection

Below the Broadband heading, right-click PPPoE Connection.

Note You might see the name of your ISP, for example, earthlink, rather than PPPoe.

On the shortcut menu, click Properties and then click the Advanced tab.

Below Internet Connection Sharing, click the Allow other network users to connect through this computer's Internet connection check box.

From the Home Networking Connection drop-down list, select the networking connection to which your computer is connected (for example, Local Area Connection).

Click OK.
The connection to the Internet is shared to your console."

Visto kosi sembra semplice ;)