Before you do anything be sure to have a complete backup of the server, this
is probably the most important thing to do!
Install a new server, run dcpromo on it, add it as another DC in your
current domain.
Install DNS, WINS and all other services you have on your current DC (if
any).
Move the FSMO roles to the new server (First in site roles), check KB 255690
for info about that.
When this is done verify that everything is running and make sure you really
have moved everything so that there is nothing you have forgotten, just to
make sure...
On the old server, run dcpromo to demote it to a member server, unistall all
services like DNS and what not.
After this wait a while and read all logs so that all attributes are really
removed from AD about this server.
When you are sure there is no problems with this and nothing on this server
is used, unjoin the domain and shut the server down.
Then your all done.
/Fredrik