How can I install Active Directory, change my domain name and still allow my existing clients to acc

The only time you would have to change the machines and rejoin the domain will be if you "drop back" to an NT 4.0 domain. Then you will need to rejoin the NT 4.0 domain. Hopefully you won't have to face that situation. The only trick is that you must change the PDC's DNS name to match the new Active Directory name prior to upgrading. So, on the PDC do an IPCONFIG/ALL to see how it is configured. Below is a sample display:

C:>ipconfig /all

Windows 2000 IP Configuration

Host name ---------------------------- apollo
Primary DNS suffix ------------------- MyCorp.com
Node type ---------------------------- Hybrid
IP routing enabled ------------------- No
WINS proxy enabled ------------------- No
DNS suffix search list --------------- MyCorp.com

But you want to call the Active Directory domain ABC.MyCorp.com. So on the NT 4.0 PDC prior to upgrade you make the change. Just to make sure you go to the IPCONFIG/ALL again and see:

C:>ipconfig /all

Windows 2000 IP Configuration

Host name -------------------------- apollo
Primary DNS suffix ----------------- ABC.MyCorp.com
Node type -------------------------- Hybrid
IP routing enabled ----------------- No
WINS proxy enabled ----------------- No
DNS suffix search list ------------- ABC.MyCorp.com

Of course, this being an NT 4.0 domain, the NetBIOS name of the domain is still ABC. When you upgrade to Active Directory you indicate that you want the AD domain to be called ABC.MyCorp.com (to fit my example). But you indicate that the NetBIOS name should remain ABC. All your machines will happily make the adjustment.

As always, testing is the best way to prepare and double-check that your plan is correct. Build a workstation to be a BDC (backup domain controller) in your domain. Then, take it off the production network and connect a hub (or cross-over cable, for that matter) to another workstation. Make sure the NT 4.0 and/or XP workstation can connect to your little domain. Promote the BDC to a PDC. Since none of this is on the production network there should be no conflicts. Then upgrade the server to Windows 2000 AD using the step hints I just gave you. Reboot the XP workstation once the AD is done and you will see the changes it makes in the configuration. But your NT and XP workstations will not require any intervention (other than the reboot) to get them to find the domain. Of course, DNS must be configured correctly in order for the XP machines to locate the domain now!