Sounds as if you're going about it the wrong way. The easiest way to share an internet connection is with a router. A wired router costs about $30. You can also get a wireless router, which costs more, but will connect computers, such as laptops, that have built-in wireless cards.
You would not use a crossover cable with a router, but rather a plain ethernet cable between the modem and the router, and another ethernet cable between each of the computers and the router. Some computers are connected to the modem by a USB cable, but ethernet is the preferred method.
As mentioned in the previous post, if the computer you're trying to connect is without an ethernet card, it must be installed. If said computer uses Windows XP, it will often find the card without additional software, but if it's an earlier operating system, the software included with the card will be needed in most cases. Ethernet cards usually cost between $15 - $20.
Crossover cables are particularly useful to hook two computers together without a network. You simply plug the crossover cable into the ethernet ports of two computers and set up Windows networking. It's also possible, if your computer is connected to the modem by USB to connect the two ethernet ports from computer to computer, though it will work better with ethernet. More