There are three things to do to completely disable it: 1. In Options -> Settings, turn off the "Launch CCleaner app window each time the computer starts" option 2. In Options -> Smart Cleaning, first turn off "Tell me when there are junk files to clean" and then turn off "Enable Smart Cleaning", then click on yes on the confirmation window. 3. In Options -> Updates, turn both options off. If you do only the first and/or second steps, CCleaner adds itself to the startup list when you launch it.