http://www.stevengould.org/index.php?option=com_content&task=view&id=15&Itemid=69
I turn off the toilet flushing sound. Disgusting.
CCleaner is fine for temp files, but registry cleaners are a hoax and cause problems. There is no evidence anywhere that "cleaning" the registry does anything at all. The notion that having unnecessary entries in the registry is "dirty", and that "cleaning" them will somehow make the computer run faster is a myth with nothing but the anecdotal "evidence" from unqualified people to provide any basis.
MS Engineers have run tests comparing the operationality of machines with "dirty" registries vs. "clean" registries, and there is no performance difference whatsoever.
There are no "rules" for the registry, only conventions. Meaning that while most software manufacturers might put certain registry entries in certain places, there is no defined rule or standard that requires them to do so. Frequently there are situations where a registry entry may be non-standard, but it is nonetheless necessary for the operation of the software that put it there. A registry cleaner comes along, deletes the non-standard entry, and the software stops working as a result. Obviously a company like Microsoft is going to be more likely to follow their own conventions than others, but maybe not. Smaller companies, and "home made" software (such as what black hat marketers might use) can do whatever they like with the Registry.
One example that I know of is that Alcohol 120% places "secret" entries in the registry under the "virtual driver section" and their only purpose is to act as a "marker" to indicate to future attempts at installing Alcohol 120% that it has already been installed and that the Trial Period is expired. This is done to prevent people from uninstalling & reinstalling the software in order to continuously get a neverending Trial Period. This is an "unconventional" use of the Registry. It was not intended to be a repository for secret codes that "mark" an Operating System has having a software previously installed. And yet they do it anyways. There is nothing to stop them. If one company is making non-standard registry entries, one should assume they all do, to one degree or another, and adapt their computer usage accordingly.
Bottom Line: Registry Cleaners present risk, and offer no benefit. They create the illusion of some benefit, but actually deliver nothing other than the sense that the User has done something positive. They have not; all they have done is exposed their system to the possiblity that a necessary registry entry has been deleted.