Unable to join computer to domain

A newly installed Windows Server 2008, adn configured as to work as
office server.

We have 10 vista workstaions, three Vista laptops connected to domain.

Internet is provided through a ADSL routers, all computers are directly configured thorugh router for the internet.

Now yesterday, I was trying to add two vista workstions to the domain, server gives error, DNS error,or user name & password incorrect, etc.

Please help me to solve this problem,

If u need any further details, I can provide?

Ben