What most people would suggest is to get a switch/router - one culprit is a USB so-called cable modem, you can get something like this
http://www.draytek.co.uk/products/vigor2200usb.html and plug in your USB cable modem into usb port, but I rather get a different cable modem with basic RJ-45 connection. On the other hand you might struggle with your USB modem and make it work if you read Linux-USB howto
http://www.linux-usb.org/USB-guide/book1.html for instance, but then you'll need bunch of NIC cards to connect all your PC's between themselves and the Linux box which would be set up as a switch/router. But again it introduces a lot of issues b/c Windows, even XP, isn't a good platform for routing setup, you can create networking loops with a Windows PC with two network cards if you are not careful enough and don't understand some of the "networking" options introduced by XP. Linux can handle routing natively with routed. So the bottom line I am trying to draw is to get a switching/routing device and go from there, than you can truely network your PC's and Linux box with Samba, so you'd be able to share files and what not on your network. Good luck.
