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I have upgraded CCleaner many times, sometimes there is check box for installing Avast and sometimes not.
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That has been what people have reported for years with the Google installs slipping by, and now also with Avast. Although they were "supposed to have fixed that". It's one of the reasons I avoid installers (don't trust them from anyone) if possible, and go the portable route.
Note that for the majority of installations (particularly reinstalling users) there is no checkbox - not because it is hidden but because there is no third party offer being made that time.
...and that would be because a registry key is created detailing not to ask for a particular amount of time that is at least when it pertains to any 3rd party Google offers, I don't know about the Avast offers.
If you forget to uncheck it then it's not the installers fault. You can use unchecky which is for average users who are not careful about reading everything and accept all defaults when installing programs.