SP2 killed WU

I bit the bullet last night and installed the 75 MB Service Pack 2 for
Windows XP. Wish I'd curbed my impatience and waited. Amongst other
things, Windows Update no longer works

The WU start page has instructions for fixing this presumably common error,
which boils down to changing 'permissions' on a particular registry key. My
system won't let me do that, even in Safe Mode. Perhaps an expert on all this
permissions stuff
can help please, as it's largely a black art to me. There must be a way of
forcing permissions to Full Control & Read?

--
Terry, West Sussex, UK