Doing an update, like I have without error for a decade+, and it says:
CCleaner v5.06 Setup
Error opening file for writing:
C:\Program Files\CCleaner\CCleaner64.exe
Click Abort to stop the installation,
Retry to try again, or
Ignore to skip this file.
None of which helps although ignoring lets the install complete but opens CC v5.05.5176 (64-bit).