I recently upgraded my systems to two HP EliteDesk 800 and other than one machine being RAID1 they are configured identically. Both are W10 pro, clean install
When I access my second machine I get a screen pop up (usually in the middle of me logging into another program and it is asking me if I want to allow Ccleaner to make changes to my machine.
I have never had this request on any other machines, ever
This is likely to do with your Windows account settings. Try right-clicking the CCleaner installer and 'Run as Administrator'. You probably won't get the message again.
Mine is also a local account (I don't even have a MS account anymore) and I don't see the UAC with CCleaner.
It strikes me as odd that you say this is often happening when you are "logging into another programme". Why would CCleaner be launching at that time? CCleaner is not any kind of malware protection.
I'm taking a guess that it may be that the login is creating temporary files and so CCleaner is launching because of Smart Cleaning?
If you have Smart Cleaning enabled try turning that off and see if the issue still happens when logging into something else.
PS. Do you still get the UAC if you launch CCleaner manually on that machine?