I spent the better part of my weekend trying to network a new PC I bought with Vista and my XP laptops. While they can be networked, it's a tricky thing. The first thing you have to do, is download the following on to your XP computers (you'll see the explanation why on the Microsoft page).

http://support.microsoft.com/kb/922120

The next thing that had me stumped was the firewalls. If you're running a hub such as Belkin, Linksys, etc., enable the firewall on your network hub and disable firewalls on your individual computers - this includes the Windows firewall as well as any 3rd party firewalls like Norton, etc.. If you're running a network without a hub, I have no idea since I have a hub but hopefully you'll be able to network right away since there's no hub involved.

Good luck.