creating a backup domain controller

Before I start off let me explain: I just started a job where their IT needs
have been ignored for some time. When they realized they were in trouble
they hired an IT guy, namely Me. Keep in mind that I just graduated with a
degree in Business Information Systems and Technologies in May. My experiece
level with MS server family is not where i want it to be.

The main domain controller in the office (also holds employee documents and
backups) is a Windows 2000 server with service pack 1. On a whim i
downloaded the newest service pack. When it installed everyone in the office
lost their connections to the server and could not log onto the domain.
Instead of wasting time trying to diagnose it, I quickly un-installed the
service pack and decided to go a different route.

I got my hands on their fax server machine and installed a clean copy of
server 2003 that i owned personally but never used. It is up-to-date and now
I want to make it a backup domain controller so i can perform my work on the
original server without any noticable difference to the users.

How do i go about starting this process? Or is there a better way to do this
that I am not seeing?

Jon-Paul