About Domains

HI,

I have 50 Windows 2000 Professional Computer and one windows 2000 advanced server with active directory installed.I am going to create two more domains one for Exchange Server and other would be child domain.I want my users can login from child domain and also from parent domain.They can select domain name from the list then they could login.How to configure two domains name in client computer.
I will be thankful if someone provides me complete assistance.

Thanks and Regards.

Anurag.