Microsoft Knowledge Base Article 814588 contains the following summary:
This step-by-step article describes how to return the computer to the state that it was in before you ran Sysprep.exe or Riprep.exe. Returning the computer to this state prevents the Mini-Setup Wizard from running when you restart a computer.
This procedure is useful if the Mini-Setup Wizard causes the original computer to stop responding (hang), therefore preventing Setup from finishing. This allows you to troubleshoot the reason that Sysprep.exe did not successfully complete. When you have completed this procedure, you can rerun Sysprep.exe or Riprep.exe.
You may also want to use this procedure if you accidentally run Sysprep.exe or Riprep.exe on a production system and you did not want to change the Security IDs (SIDs) or any of the configurable options that the Mini-Setup Wizard prompts you for when you next restart the original computer.