Did you ever wonder why Windows does not include a registry cleaner as a Windows native tool? Could it be that Microsoft may feel its usage is not necessary and that it has the potential for being harmful?
So called "registry cleaners" can aggravate a situation and take steps we are many times not manually aware of. If a user lacks the skills to clean the registry manually, he should not be doing so automatically. It is better not to resort to such programs in cases in which you don't know what is happening with the proposed fix and the registry. Otherwise, a change to the registry can make a system unbootable by one mistake. In most cases the damage done is not seen or even attributed to the cleaning. However, it might manifest itself with an application that is not behaving as it used to, or with a program feature that no longer starts as it should.
There are no gains to be had from using a registry cleaner and the risk is great. Registry cleaners are not able to distinguish between good and bad, other than to delete keys which are not being used and are obsolete. Those keys may be needed by the operating system or by programs later on.
If your computer is running slowly, and if Windows disk cleanup and tools such as free CCleaner Slim Build (without using the registry component) do not help, try posting at one of the forums that use trained experts to review diagnostic logs. They will walk you through manual cleaning, or guide you through running a free cleanup program.