CCleaner tops my list of free registry cleaners for several good reasons. Aside from being completely free and working with all versions of Windows, two additional things really stand out.
For one, I've never had CCleaner cause a problem in the Windows Registry, which some less-well-done registry repair tools do on a regular basis. And two, because it's optionally available in a portable format (i.e. it doesn't need to be installed).
You need to be protected from all those nasty Spywares, Malwares and Ransomwares, luckily, Ccleaner will remove most of them