I've played around with registry cleaners for a long time and believed in them, by and large. That was until using one, Advanced System Care, required me to do a clean re-install of my Windows XP x64 operating system if I wanted to continue accessing Microsoft updates manually. [No such problem was caused by ASC on a x32 bit operating system.] That experience made me think twice about them and caused me to do a little research. To my surprise I found the cognoscenti were not only divided but that some very knowledgeable commentators were opposed to the use of registry cleaners. Here is a brief summary of what I found:
BASICALLY ANTI: Ed Bott ; Leo Notenboom ; Blair, relying on Mark Russinovich and Bill Castner.
IN THE MIDDLE: Scott.
BASICALLY PRO: Fred Langa ; Bob Rankin.
IF YOU WANT ONE WHICH IS THE BEST? For comments and reviews see Steve Bass of PC Advisor, August 2008 ; Top Ten Reviews 2009 ; Mark Anderson ; Neil J Rubenking of PC Mag, August 2004 plus the two sites above which are basically pro and Leo Notenboom's.
My conclusion was that if you are going to use a registry cleaner make sure it is a conservative one like CCleaner's. Others can have unintended and unexpected side effects.