After upgrading to Windows 10, running the CCleaner update installation didn't work for me. Running the file as administrator didn't work. Setting up a separate administrator account and running it from there didn't work either. Here's the message that I kept getting --
Clicking on "Retry" didn't work, and clicking on ignore gave a result that said the installation was successful, but failed to update from the old version.
Here's what worked for me:
1) Open Windows Explorer (type "file explorer" in the search bar and click on the App at the top of the screen),
2) Navigate to C:\Program Files\CCleaner, and open the folder. In the right-hand pane, you should see something that looks like this --
3) Right-click on a blank spot in this area, and from the drop-down menu, select "New > Folder." Name it "old files," if you like.
4) Move the existing files to the "old files" folder. In this way, you haven't permanently lost the current version. You can move the files back to where they were before, if need be. You have also retained the general path that Windows sees when it looks for those files. Now it should look like this --
5) Close Windows Explorer.
6) Re-open Windows Explorer, navigate to "Downloads," find your newest installation file, right-click on it, and select "Run as Administrator."
7) ¡Viola! You now have the latest and greatest version!