I haven't a clue about Norton Utilities. I gave up on it years ago after it went through a phase of causing more bother than it was worth, resulting in enough calls to sort out client machines to convince me to avoid it at all costs.
If you can boot from a floppy and use the change directory command to get to C: then you can simply rename the existing registry file c:\windows\system.dat (you need to remove certain attributes first) then copy your registry backup from a DOS prompt along the lines of copy c:\registry_file_name.1st c:\windows\system.dat
This assumes that you are talking about an entire registry backup, not just certain branches of it.
There is loads of information on the web about this kind of thing. One link to get you up and running is click here