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Assuming you have a BDC on the domain, a general procedure for rebuilding a PDC is:
1) Run a full backup and 'rdisk -s' (copy the security hive elsewhere). This is just a precaution. You would normally not use it in the rebuild.
2) With the PDC still online, promote an existing BDC to PDC.
3) Remove the original PDC (now a BDC) from the network.
4) If you failed to do step2, promote a BDC to PDC before completing the next step.
5) When you reinstall NT Server on the original PDC, select the option to install it as a domain controller in an existing domain. The machine must be on the network and able to reach your current PDC, during this step.
6) When the rebuild of the old PDC is complete, promote it to PDC.
If you failed to follow these steps (e.g. you installed NT Server as a PDC in a new domain), you have actually created a new domain with the same name as your original domain. The SAM will have only the default accounts, because it is a new domain.
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