Did you try deleting the domain and setup for the XP workstations, and reinstall it afresh? You'd have to do it on each one but I think it might work without having to reload the entire OS.

You can try this procedure:

1. Right-click "My Network Place", "Properties" and enter the properties for the "Local Area Connection" on your system.
2. Highlight "Client for Microsoft Networks" and click "Uninstall"
3. Reboot the system when prompted.
4. When the system comes back up re-enter the properties for "Local Area Connection" and click on "Install"
5. Double-Click on "Client" and select "Client for Microsoft Networks"
6. When the Client is reinstalled, reboot one last time and you should be able to join and browse the domain.

You'll need to do that on each workstation if indeed it works.