Can anyone give me some information about how the login process works in NT4.0

Environment
1 PDC
2 BDC (file server, Oracle server)
1 Mail server

Problem:

Clients appear to be only checking authentication with one of the BDC's only

Description of problem:

If I take the one BDC offline for maintenence. The clients won't logon or run the login scripts. I receive an error that the login scripts could not be located [server name] or even can not find PDC on network using cached profile (or something like this)

I thought that the PDC would take the responsibility of authenticating and applying the login script. Do I have a hidden problem with the PDC?

I was told that it could be a registry setting (???)

Anythoughts to why this would be happening?

!!!! YES WE ARE MIGRATING TO 2003 NEXT MONTH !!!!

But I need to take the BDC offline - we are redesigning / restructuring the domain and this server will become a first DC for 2003

*** A FRESH START ***