Windows has a system Registry that records everything you do to your machine. This includes files you add/remove to the machine, links to all said files, and other registry values for these files. This includes the installation and settings for all your software.
As you add/remove files and software to your machine, the registry grows. (Yes, the registry actually gets bigger even if you remove something.) This is in part that the registry will not remove links to files that no longer exist. The registry size also affects the speed of your computer, and can slow you down when it attempts to "touch" files that no longer exist.
A Registry Cleaner can greatly improve the speed of your computer by removing entries that are no longer needed and/or are no longer valid.
You may download a Registry Cleaner using the link below: