Microsoft Knowledge Base Article 315136 contains the following summary:
This step-by-step article describes how to run the semantic checker on the Active Directory database.
Unlike the file management commands, which test the integrity of the database with respect to the ESENT database semantics,
the semantic analysis analyzes the data with respect to Active Directory semantics. You can use this process to generate reports
on the number of records present, including deleted and phantom records.
The Windows 2000 Directory service opens its files in Exclusive mode. This means that the files cannot be managed while the computer is operating as a domain controller. The first procedure is to boot your server into Directory Services Restore mode.