[smoore]

This would be better answered in Computer Technical Help but here goes:

You need what's called a router. Many manufacturers make them. Linksys is a solid performer in the home market at a good price.

Most cable systems require you to buy another IP address for an additional computer. The router lets you get around this by using "MAC address cloning". Follow the instructions in the manual for this BEFORE you plug the router into the cable modem.

edit: it's also possible to use windows ICS (internet connection sharing) and another NIC in the "gateway" (not brand) machine. It's simpler to just buy a router.