CCleaner starts when the computer starts even when the option is unchecked in latest 64bit version.
It's a common misunderstanding.
"Run CCleaner when the computer starts" runs a one-off clean in the background and then closes CCleaner. If you uncheck it then it doesn't run that background clean at startup.
If your CCleaner is starting and then continuing to run then that is the Smart Cleaning.
Smart Cleaning monitors how much junk is building up on your computer, and deletes it when it reaches the level set. (You can change that level in CCleaner Pro).
So it has to run all the time in the backgroud to monitor how much junk is building up as you use your computer.
You can turn Smart Cleaning off if you don't want it running.</span>