|Win XP contains an informative walkthrough in it's Help & Support menu.|
Before buying anything you need to define what you wish to achieve, and to consider what future expansion you may desire.
Firstly, are you looking only to share files between the two systems and/or play LAN games?
Are you looking to share an internet connection? If so what type of internet connection is it?
Next. Do the two computers already have either RJ45 ethernet ports or perhaps wireless network cards?
If the two systems already have inbuilt network cards and are ready for wired networking then the only thing you need to buy is a crossover CAT5 cable. That will allow you to establish a network between the two systems for filesharing purposes.
Goin' Fishin' (Some day)