If CCleaner is not already running, open Start Menu by clicking on Start button or pressing Windows Key on keyboard.
Window XP users should choose All Programs, find CCleaner folder and click CCleaner:
Windows Vista and 7 users should type "ccleaner" into Search box and click CCleaner in search results.
In Windows 8 and 8.1 Start screen, type "ccleaner" (you do not need to open Search charm) and click it in the results. Touch-screen owners can just scroll to the end of Start screen and tap CCleaner.
As CCleaner needs administrative rights for cleaning temporary files and registry, User Account Control will pop up. Click Continue (Vista) or Yes (7 and 8).
Since version 3.19, you can disable User Account Control prompts in Advanced Options of CCleaner. Open Options tab on the left and click Advanced. Then enable the Skip User Account Control warning option.
After clicking Registry tab on the left, click Scan for Issues button. Those who want to be extra safe, can untick the Unused File Extensions check box: only very few programs run into minor trouble after cleaning unused extensions.
You will probably get a quite long list of issues when running CCleaner for the first time. Click the Fix selected issues... button:
CCleaner will ask if you want to backup changes. This will create a backup of issues deleted from Windows registry. Click Yes here:
Save the backup file to your My Documents folder (Windows XP) or Documents library (Windows Vista, 7 and 8/8.1) by clicking Save:
Next, click Fix All Selected Issues and CCleaner will remove invalid entries from Windows registry:
After cleaning is complete, click Close button:
But you are not done yet!
Repeat the process by clicking Scan for issues button again. Most probably you will see another list of issues. Because CCleaner does not cross-reference previously deleted items, there are usually more useless entries in Windows Registry. CCleaner finds them only during second scan. So repeat the process of clicking Fix selected issues, backing up changes to your My Documents folder or Documents library and clicking Fix All Selected Issues and Close buttons.
Click Scan for issues... for the third time and you should see "No issues detected" message. If not, repeat the cleaning process and do another scan until you see the "No issues detected" message.
As Windows Registry is a file, it gets fragmented during cleanups and program installation/uninstallation. This might hinder computer performance somewhat, so please see the Defraggler tutorial for information on how to set boot-time defragmentation of most important system files.