The Windows Registry is a set of files that contains settings for your Windows PC - such as what program to use to open HTML files, when your antivirus program should run, or what desktop background you use. You can read more about the Windows Registry at Wikipedia.
Over time, the Registry can become cluttered with missing or broken items as you install, upgrade, and uninstall software and updates. CCleaner can help you clean out the Registry so you'll have fewer errors. The Registry will run faster, too.
To clean your Registry:
Note: To scan one type of Registry entry in the Registry Integrity list, right-click it, and then click Scan [Registry Type].