CCleaner prompts you to back up the Registry first.

Click Fix Selected Issues to fix the Registry problems.

You can review the current set of excluded files, folders, and Registry entries in the Exclude section of Options > Exclude in CCleaner.

If you want CCleaner to skip any of the issues it's found (permanently), right-click the issue, and then click Add to Exclude List.To save the list to a text file, right-click anywhere inside the list, and then click Save to text file. You can review the list of issues once the scan finishes (this is really for advanced users).You'll see a progress bar and a list of potential issues. We recommend you leave all the items selected. You can then select the items under Registry Clean you want CCleaner to scan (they are all checked by default).In CCleaner, click the Registry icon at left to view the Registry Cleaner menu.Clean your Registry with CCleaner for Windows: CCleaner cleans out the Registry to help prevent errors from occurring, which can help it run faster. Over time, the Registry can become cluttered with missing or broken items when installing, upgrading, and uninstalling software or updates. Why CCleaner cleans the Windows Registry: You can read more about the Windows Registry on Wikipedia. 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. To skip to instructions for Registry > Registry Cleaner, go to Clean your Registry with CCleaner for Windows further down this page.

This page explains what the Windows Registry is, why CCleaner cleans it, and how to clean it.

Ccleaner clean registry