Remote Desktop Licensing Problem

I am trying to remote desktop from an XP PRO machine into a terminal server running server 2000. I keep on getting an error message that ?The remote computer disconnected the session because of an error in the licensing protocol? It is my understanding that XP PRO does not need a CAL to use the terminal server. All of the other computers can remote desktop without a problem, but this one cannot. I do not think that we have a license server setup anywhere, but the admin that I replaced does not think that we have ever had one. I have deleted the registry key on the XP machine several times, it has not helped. Can anyone help me with this?