While hacking the registry for a future tip, I destroyed my installation so thoroughly that NT would not boot and setup would not repair it. The NTFS file system was still intacted but the registry was so damaged that setup did not recognize a valid installation. While I could have restored, I would have lost 4 hours work.

To fix it, boot to an alternate install of NT and expand the each hive on your ERD (or original %SystemRoot%\Repair folder)
to the original %SystemRoot%\System32\Config folder:

expand c:\winnt\Repair\Software._ C:\WINNT\system32\config\Software
expand c:\winnt\Repair\Software._ C:\WINNT\system32\config\Software.sav
expand c:\winnt\Repair\Default._ C:\WINNT\system32\config\default
expand c:\winnt\Repair\Default._ C:\WINNT\system32\config\default.sav
expand c:\winnt\Repair\SAM._ C:\WINNT\system32\config\sam
expand c:\winnt\Repair\SAM._ C:\WINNT\system32\config\SAM.sav
expand c:\winnt\Repair\Security._ C:\WINNT\system32\config\security
expand c:\winnt\Repair\Security._ C:\WINNT\system32\config\security.sav
expand c:\winnt\Repair\System._ C:\WINNT\system32\config\system
expand c:\winnt\Repair\System._ C:\WINNT\system32\config\SYSTEM.ALT
expand c:\winnt\Repair\System._ c:\WINNT\system32\config\system.sav

Here is an example of the response from
expand c:\winnt\Repair\Software._ C:\WINNT\system32\config\Software

Microsoft (R) File Expansion Utility Version 2.50
Copyright (C) Microsoft Corp 1990-1994. All rights reserved.
Expanding c:\winnt\repair\software._ to C:\WINNT\system32\config\Software.
c:\winnt\repair\software._: 1242580 bytes expanded to 6377472 bytes, 413% increase.

If the files required to boot your alternate install are damaged or missing, you will need a boot floppy (see tip 012). After doing this, I was able to boot and in my case, I was fully recovered. You may need to repair boot records using your ERD.