What do you connect from your computer to your Xbox 360 to get Xbox live?

You have to have an ethernet router. The modem plugs into the router's source port. The PC and Xbox 360 (and any other peripherals) can plug into any of the various connectivity ports. Any Windows OS from 2000 to Vista will detect the console as an extension.

You have to into your PC connection setup on your console, get a unique ID code, and it will auto-configure your PC to communicate with the console. You can then browse your PC's media databse for music, videos, and pics to browse from your Xbox 360.

A media center edition of Windows will seamlessly integrate with your Xbox 360's Media Center upon the detection of the network.