Did you run the XP Network Setup Wizard on all machines? If you ran the
wizard, there's a place where it asks about your Internet Connection. The
wrong choice will block machines from seeing each other or at least the XP
machine. For most networks, the correct response is the "Gateway" response.
When you run the wizard, be sure any security software is turned off on all
machines. If you have anything besides ICF, any other firewalls or software
designed to block ads, popups, even antivirus software, turn them off on all
machines before running the wizard.