You will see this error message if you are trying to install CCleaner without being logged in as an administrator:
Or, if you are running Windows 7 as a non-administrator, you may see this error message:
Solution #1 (pre-Windows 7 error): The first dialog box may appear if a previous version of CCleaner is running when you run the installer. Wait a few seconds, and then click Retry. If this does not work, try solution #2.
Solution #2: Log in as an administrator and then run the installation program, or contact your system administrator for assistance.
Alternative solution: Use CCleaner Portable which you have installed on a USB drive using another computer. It may have limited functionality if you are not using an administrator account.
Note: Once CCleaner has been installed, you can run it without being logged in as an administrator, but it may not be able to clean certain files and folders.