how to add Vista Ultimate to Win 2003 Server R2 SP2,?

Dear Concern,

We have windows 2003 server R2 with SP2,I would like to add vista Ultimate system to a Domain Controller,
While adding to domain i am getting the following errors: An Active Directory Controller for the domain Could not be contacted.
I am able to ping to the server,Domain Name is correct what would be problem.
Appreciate your Answer.