Restoring a crashed server by reinstalling NT and restoring the IS and Directory Service (DS) databases doesn't restore the IMS configuration information because the IMS keeps its configuration in the Registry.

This behaviour is a pain in the neck because you must manually restore the IMS settings to their original state. Fortunately, tools are available to automate this process. The Imcsave, Imccopy, and Imcrstr tools (all available in the server\support\utils\i386 directory on your Exchange Server 5.5 CD-ROM) let you back up and restore the IMS configuration and directory structure. In short, everything you need to clone a failed IMS.

See the Microsoft article "XFOR: Imcsave Utility" at http://support.microsoft.com/support/kb/articles/q195/8/04.asp and the readme.doc file in the utility directory for details about how these tools work and what to do with them.