How do I stop Windows 10 from blocking my installation of CCleaner?

Trying to use an outdated version of CCleaner

If you have an extremely old version of CCleaner (ie: version 5.45 or earlier) then it is not compatible with recent versions of Windows 10.  Please download the latest version of CCleaner from https://www.ccleaner.com.  

Windows Store restrictions

You may also experience difficulties installing even newer versions of CCleaner if you have set Windows 10 to only permit software purchased from the Windows Store.  If you get a message when you try to install CCleaner that says "The app you're trying to install isn't a verified app from the Store" or "You can only install apps from Microsoft Store" then you may need to make some changes to your Windows settings to permit you to install software like CCleaner that you have bought or downloaded for free from elsewhere.

You can do this by going to Settings > Apps > Apps & Features in Windows and then for the "Installing Apps" option, change your setting from Allow apps from the Store only to either Warn me before installing apps from outside the Store or Allow apps from anywhere, if you would prefer not to be asked for permission each time.

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If your setting is Warn me before installing apps from outside the Store then you will see this message "The app you're trying to install isn't a verified app from the Store".  You can just click Install Anyway to continue.

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Note: Behavior of this sort can also occur when using Windows 10 in S Mode; some devices will ship with the operating system in this mode to begin with. You can find out more about S Mode and, if desired, how to disable it here. CCleaner cannot be installed while Windows 10 is in S Mode.

Temporary false positive

When new versions of CCleaner are released, they are downloaded by millions of users on the first day.  Although presently all new CCleaner versions go through cybersecurity testing before release, some antivirus vendors may temporarily flag these new CCleaner releases that they have not seen before as "risky" until their analysts have had a chance to review them in detail.

Depending on the quality of your antivirus solution provider, they will generally correct any such "false positives" in 12-72 hours.

Alternatively, you can download the alternative "slim" build from here: https://www.ccleaner.com/ccleaner/builds as this will often, but not always, prevent such "false positives' from occurring.