How do you restore an older version of the registry in Windows XP?

I run Win98 on one of my computers and just had to restore an older
version of the registry after letting a reg cleaner program screw it
up. In Win98 that is done from the Dos prompt. But I know XP dont
have a true dos prompt, so how would you do this restore? I dont need
to do it, just curious.

I asked in a Windows98 newsgroup (crossposted from this XP newsgorup)
and no one could answer it.

And fo the heck of it, would this also be the same method for Win2000,
Vista, and Win7? (I also have Win2000 installed).

Thanks