The Windows registry is a database in which Windows keeps track of relationships between hardware, software and the operating system. Over time the Windows registry will become saturated with redundant files and broken links that will slow down youw Windows operating system and make it sluggish.
A registry cleaner is a piece of software that runs on the Windows operating system, issues involving the registry could cause you to lose your personal data, or make the computer completely unusable, you need to be very careful with it, all of the registry cleaners will allow you to store a back up copy of the old configurations before cleaning it. The program will scan through the registry keys and point out any issues which it finds, the most usual problems are registry keys pointing to non existent files already deleted, or registry keys left by long gone uninstalled software as well as pointers to missing DLLs, listings of no-longer-valid file extensions, invalid paths, invalid ActiveX Controls, and much more.
A good registry cleaner will attempt to fix those keys, leaving your registry slimmer and your computer prepared to run free of junk once again, some cleaners will also compact and defrag the registry making it easier and quicker for Windows to access the keys.
List of free Windows registry cleaners
ACleaner: This two-button Windows Registry cleaner lets you search for problems and then fix them. You can choose which Registry sections to include and restore a backed-up version of the database if you encounter any problems.
AusLogics: Auslogics Registry Cleaner offers a simple wizard-based interface. You can choose between quick and advanced scans, and undo any previous changes. If you choose to run an advanced scan, the program lets you select the categories you want to target.
Eusing: Eusing Free Registry Cleaner will repair registry problems with a few simple mouse clicks, this is one of the easiest to use registry cleaners, there is a pop up window when you start the program asking if you would like to donate.
Comodo System Cleaner: Comodo System Cleaner is a powerful suite of utilities and tools that allows you to make your Windows based computer faster, more efficient and more secure, this program is a full suite that includes a registry defragger.
Wise Cleaner: The free version of this registry cleaner has no registry compactor (defragger), but otherwise it quickly identifies orphaned registry keys and makes them easy to fix.