How do I reinstate my old PDC back into the Domain as the PDC?

Start up the old PDC machine

Logon to the machine as an Administrator

Start Server Manager (Start - Programs - Administrative Tools - Server Manager)

The machine will still be described as a Primary Domain Controller, however its icon is just a wire frame and it is not acting as a PDC, i.e. it does not authenticate logons

Select the machine, and from the Computer menu select "Demote to Backup Domain Controller"

Click "Yes" to make the change

Once the machine is a BDC, click on the computer again and from the Computer menu select "Promote to PDC"

Click "Yes to make the change

Server manager will then automatically demote the temporary PDC back to its BDC status and promote this machine back to the PDC