Home networking software?

By some bizarre fluke I've been able to set up a Microsoft Windows home network. Two computers are connected together and can share files and the internet, and all that jazz. Now I've got a third computer and I want to connect it. The catch? It's ancient and it's running a version of Windows XP called XP Mini, which (whilst only using 30mb ram and 500mb of hard disk space) has absolutley no networking facilities. So I'm looking for a piece of software that can connect this computer to a Windows Network. Will Network Magic do the job? Do I have to be using that to manage my network? Is there any other software that can? I don't pretend to know an awful lot about networking, but does anyone have any good suggestions? I'm not using wireless. It's old-timey cable networking.