There are backup copies of the registry in the
\windows\system32\config\regback folder
You can try replacing the ones in the config folder with the ones from the
regback folder.
Rename the ones in the \windows\system32\config folder first
Change directory so you are in the \windows\system32 folder
ren default default.bak
ren system system.bak
ren software software.bak
ren sam sam.bak
ren security security.bak

Then
copy c:\windows\system32\config\regback\default
copy c:\windows\system32\config\regback\system
copy c:\windows\system32\config\regback\software
copy c:\windows\system32\config\regback\sam
copy c:\windows\system32\config\regback\security

All five registry hives should be replaced at the same time even though you
are only having issues with one of the hives, they are a unit, they are not
intended to be replaced individually, there may be issues later if one of
the hives is rolled back and not the others.