NTREGOPT, or NT Registry Optimizer, is a Windows Registry defragmentation utility.  Over time as entries are added and deleted from the Windows Registry, gaps are created. These gaps are unused space that takes up unnecessary disk space and reduces the overall performance of accessing the Registry.  NTREGOPT rebuilds the Registry so that it is compact and does not contain any extra spaces.

ERUNT, or The Emergency Recovery Utility NT, is a utility that can be used to backup and restore the Windows Registry.  This tool has the ability to do a complete backup and restore of the Windows Registry, including the security hive so that permissions are properly backed up and restored.