Windows NT don't have active directory, you will need to upgrade your domain to windows 2000.
To do that:
1 - install another windows NT on a Virtual Machine.
2 - Promote the VM Windows NT to a BDC
3 - make sure that you have replicated it with your PDC
4 - Promote the VM Win-NT to PDC so you can try the migration process without break service or lose the service :)
5 - In the VM promoted PDC begin to install windows 2000 upgrading it from NT to W2k.
6 - Now we have a DC in windows 2k make sure everything is ok and that the users still have connectivity migrate all information from the old server to the VM.
7 - upgrade the old server to windows 2000 also.
8 - when finished migrate the PDC server and the Time server, GC etc from the VM to the new server.
9 - Migrate services from VM to physical machine.
10 - Upromote the VM Domain controller to stand alone server
11 - remove the VM from domain and disconnect.
13 - Upgrade to win 2k3 :)