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Restoring the system hive without an ERD
If you don’t have a recent copy of the system hive saved to an ERD, you have two options for fixing the damaged system hive on your server: Fast Repair and the Recovery Console.

Using Fast Repair
You can run Fast Repair from the Emergency Repair Process screen described above, but before you do, make absolutely sure that you have no other choice. If you run Fast Repair on a Windows 2000 domain controller, Windows 2000 will activate the system hive as it was when you first installed the operating system.

Fast Repair looks in %systemroot%\repair for the requisite files, but these files won’t have been updated to include any additional programs you installed on your server. For a domain controller, this includes things such as Active Directory. When Fast Repair finishes, you must reboot and reinstall all necessary software and drivers. When you finally get the domain controller going again, be sure to back up the system state daily so you can restore the Active Directory from Directory Services Restore Mode.

from: http://asia.cnet.com/itmanager/netadmin/0,39006400,39117840-3,00.htm