Bebbspoke,
The checked Avast box is clearly on the first screen. All you have to do is uncheck it. I've been using CCleaner since it was in Beta and called Crap Cleaner. That was before the Piriform pear was in the upper right had corner of the program.
This is a great program and I only need the basic mode. That said, the people who created CCleaner and the ones who keep it constantly updated have to make money. If they don't CCleaner will just fade away.
BTW, even if you don't install Avast an Avast Registry Key will be dropped. I attached a screenshot of it. If you are not familiar with the Registry just leave it. Otherwise, you can delete the key if you want to.