What OS are your client workstations using? If they are Win9x or NT4, then the answer to your problem will not be easy. Let me explain. In a Windows 2000 domain with Active Directory, there are 5 FSMO's (Flexible single masters of operations). One of these is the PDC emulator. The first domain controller, by default, becomes this PDC emulator. At this point all win9x and NT4 workstations will ONLY authenticate to that server.
Therefore,when your main server is down, you will need to do several things. (How long will the server be down? 30 minutes, 8 hours, 7 days? How long?) If it is only a short time, then you should not need to do anything. First make certain that all workstations have authenticated before you 'down' the main server. If the server will be down more than a single day, you will need to use one of the other 2000 domain controllers, and promote it to the PDC Emulator. IF you know what you are doing, it is fairly easy. Then you will need to shut down each workstation for a few minutes and bring them back on line and they will find the new PDC emulator.
In a 2000 domain environment, there is ONLY one PDC emulator and ALL down-level clients will only authenticate to that server. If the clients are windows 2000 pro or XP, they will not have any problems.
Hope this helps. Best of luck.
Michael R.