When ADSM Backs up the registry it  leaves a copy of it on the HD under
C:\adsm.sys

 Copy x:\adsm.sys\registry\<computername>\Machine\*.*
C:\winnt\system32\config\   ENTER

and then
 Copy x:\adsm.sys\registry\<computername>\Users\*.*
C:\winnt\system32\config\   ENTER
x = the original C: drive.
this will work fine if you can boot from a temporary drive.
I take it you are trying to test a restore of the NT system drive???