Restore Registry

Roomate's PC.
How do I restore a registry from DOS?
Scanreg /restore just hangs.
Too long to explain right now except that the system hung , while in System
Information while doing a Registry Check. I wanted to back up the Registry due
to a problem with booting due to the MB and/or the PSU.
Now, upon bootup, the OS demands that Scandisk be run, including the Surface
Test.
It will do the same thing while selecting to boot in safe mode.
In DOS, I cannot find a system.dat or a user.dat in C:\Windows.
I can, however, find the five .cab files in C:\Windows\Sysbckup.
It's late and I am too tired to do the searching.
Thanks.