Registry Cleaners, not recommended
In other words, the registry contains all the operating system's knowledge of a computer's configuration, hardware devices, installed software and location of the device drivers.The Microsoft Computer Dictionary, Fifth Edition, defines the registry as:
A central hierarchical database used in Microsoft Windows 98, Windows CE, Windows NT, and Windows 2000 used to store information that is necessary to configure the system for one or more users, applications and hardware devices.
The Registry contains information that Windows continually references during operation, such as profiles for each user, the applications installed on the computer and the types of documents that each can create, property sheet settings for folders and application icons, what hardware exists on the system, and the ports that are being used.
Modifying registry keys incorrectly can cause Windows instability, or make Windows unbootable.
No registry cleaner is completely safe and the potential is ever present to cause more problems than they claim to fix. Registry cleaners will always find errors, even on a freshly installed operating system.
Windows is a closed source system, developers of registry cleaners are not working on definitive information, but rather empirical knowledge. Automatic cleaners will usually have to do some guesswork.
If you do have a problem that is rooted in the registry, it would be far better to edit only the specific key/s and/or value/s that are causing the problem. Always backup the registry first!
The manual changing of one registry entry is less likely to produce the unfortunate consequences of allowing an automated product to make multiple changes simultaneously.
If you do not have knowledge of the registry, then you would be better off leaving it alone, and definitely not placing blind trust in a program to do the job for you.
Registry cleaners which consistently find lots of stuff to "fix" should give someone cause for pause.
Microsoft support policy for the use of registry cleaning utilities
Why Using a Registry Cleaner Won’t Speed Up Your PC or Fix Crashes
Guide to Registry Cleaners and Optimizers
