About Two Primary Windows 2000 Domains

Hi,

I have a question there are two windows 2000 primary domains and with all domains 100 users are connected.Both the domains are on the different subnets so there is no communication between them.Then if company wants to remove one domain and transfer all the users in the single domain then how to transfer all users in the main domain.Because it is not possible to go to every computer and registered the main domain.
Is there utility from microsoft or any other vendor which transfer all the users in the domain which going individual computer.

Regards.

Anurag.