I'd recommend using this free application:http://www.321download.com/LastFreeware/
open it to the registry cleaning button on the left, then tools>registry cleaner.
It is a conservative yet very effective tool.
Follow directions, and the backup will be one of the things you do want it to do.
All registry errors will be explained, and old entries most certainly can be deleted using this utility.
It is best to do this, and the other actions allowed with the program, from the point of view:
I'm learning. I need to know this part of my OS to be confident.