The simplest solution I found that has worked now for multiple re-boots is to use SysInternals Autoruns - https://docs.microsoft.com/en-us/sysinternals/downloads/autoruns After starting Autoruns in the "Everything" tab, type ccleaner in the filter box and uncheck CCleaner Update in the Task Scheduler section.
Yeah, yeah, we get it, we don't need six posts to tell us.
Just created an account to say that, as for many other users it seems, Ccleaner is banished from my computers until Pifiform provides a reliable solution to this mess.
It'll remain a dark spot in the history of a long trusted software, though.
Bye bye, Piriform.
CCleaner 5.45 is SPYWARE.