Are you running in Admin rights or normal user?
I have a sneaky feeling there has been a change in networking side of things on the XP machines. Sometimes even a dialog box popup which you click yes to get rid of it can have far reaching consequences. If your in normal user rights it will stop you doing changes.
I have no doudt there will something on
Microsoft TechNet UK: Resources for IT Professionals
Its quite hard diagnosing probs like this without getting your hands on the machine and just running through all the network and security menus to make sure everything looks right.