Assuming you know the name of the file the registry was backed up too - it'll be something like backup.reg
I think you right click on it and select the Merge option from the pop-up menu. I say I think that's the way to do it because whilst I know this works if you want to add a registry key but I'm not 100% sure that it will replace any changed entries.
In any event it shouldn't do any harm to your system to try. To be honest your best bet might be to just re-install Norton and see if it sorts itself out.
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sj