if you know what part of the registry you need to edit, you can take your hard drive to another computer ( xp or better). connect the hard drive to the computer, then go to the hard drive and load the branches that you need into the registry editor in the current computer, do not forget to save them again once you are done.
I think the registry is in the folder \\windows\system32\config
there may not be anything there when windows is running, when you shut down windows there are some files w/o extensions system, software ...etc
these are the registry.
If your registry is that bad especially from a virus attack, it may be better to do a wipe and reload
you can always do a parralel install, installing xp to another folder.