Corrupted Hive

Microsoft provide a long explanation on how to fix this .. when the system is unable to boot up on account of a missing or corrupted registry.

This gives a tedious method of booting from a XP cd to the recovery console, and then copying files etc.

If you have the UBDW or Bart PE, you can shorten this process considerably.

Boot up in Bart PE.
Navigate to the System Volume, and find a restore set for a date earlier than when the error occurred
eg: "_restore{87BD3667-3246-476B-923F-F86E30B3E7F8}".

Copy the following set to c:\windows\system32\config 

_registry_user_.default (Addendum Note: Notice the period (".") before the word default)
_registry_machine_security
_registry_machine_software
_registry_machine_system
_registry_machine_sam

Now reboot into the recovery console, cd into the config directory, and rename the existing files to security.kpp, software.kpp, etc

And then rename the _registry* files to the last word in each file name.

Now restart Windows ( in safe mode), and select another restore point.  And that's it. Fingers crossed.