NT and 2000 Active Directory

Hi, We are running a LAN with NT 4.0 server as the primary server with one backup NT server and another NT server attached. I installed a Windows 2000 server which installed as another server on the network. I haven’t installed Active directory yet. I would like for the 2000 server to be a backup server. Is it possible to make this 2000 server a bdc (backup server) without installing Active Directory on the NT primary server, also is it safe to install Active Directory on the 2000 server (gathering information from the primary NT server) while users are logged into and working from the NT server? In other words will it interrupt the users in anyway?

Please help. thank you