Windows XP Service Pack 3 – Access Denied

I have several computers, all running Windows XP service pack 2. I was able to install Service Pack 3 on two of them, but the others get “Access Denied” and roll back to their previous (I hope). I’ve tried everything on Windows web site, but no change. I can tell from the log that it is failing on sprecovr.exe, but I can’t even find an entry for that program in the registry. And yes, I am the administrator on all of these machines.