To enable the root user you need to do the following.
1) Log Into The System With Your Administrator Account (the account you install updates with)
2) Run NetInfo Manager (Applications > Utilities > NetInfo Manager)
3) Select Security > Authenticate
4) Type Your Password
5) Select Security > Enable Root User
6) Set A Password In The Window That Comes Up.
In terms of installing, you should just need to put the OS install disk into the computer and reboot while holding the C key down.