CCleaner like Avast never make the job for me, I have always told people to stop using it.. but user need proof.
Even with ton of subject "I have removed X with CCleaner and now I.."
Even with ton of subject "I have removed X with CCleaner and now I.."
Anecdotally, I've seen CCleaner delete way too many false positives in the Registry, breaking applications, (and people have never heeded it's warning to properly backup the Registry), and worse entirely corrupt Registry Hives, breaking Windows.
The Hive database format of the Windows Registry was built to be read-mostly/write-rarely and doesn't survive well to active surgery, especially not "I run CCleaner once a week with all the options checked". Like I said, I've seen it corrupt entire Hives from too regular operation.
I'm also of the opinion that some of that "Windows slowdown" that these users complain of is a snowball impact of too much Registry surgery leaving sadly deteriorated/badly optimized for reading Hives behind, but that's mostly a hypothesis I have not scientifically proven.
[1] I kind of forgive people still running Avast out of habit from bad old XP days (not everyone got on the Microsoft Security Essentials train as fast as they could, and that was as much a marketing/awareness problem), though as knowledge that Windows Defender exists spreads there are increasingly fewer excuses to still run Avast.