How to join a domain from win xp workstation to win 2003 server over VPN

Hello everyone
When I try to join a domin over VPN I get an error RPC is not available. So it does not allow me to join the domain.

Windows XP workstation
Windows server 2003 (I added the users)
VPN connections are ok and can ping the server while connected and can access files.
I guess the question is: How to join a domain over VPN?
Thanks for any input