I just downloaded CCleaner from piriform's ccleaner website. There was no box to check or uncheck and yet I got Avast installed.
Now, I used avast for a LONG time. It wasn't until about two years ago that it started being a problem. Lower performance, bloated application with WAYYYY too much feature creep, and worst, it was missing viruses that other applications were detecting. If you are an anti-virus program, I don't care if you can defrag my hard drive while baking a cheesecake, if you can't stop viruses, you're gone. I was really not happy about this because I loved Avast and, now that I'm living and working in Prague, I'm close to their really amazing office that I still might visit during an open house. However, they have really screwed up their product and I won't use it in its current form.
Now, finding this completely silent installation, I not only don't want to use Avast, I'm actively pissed off. What kind of legitimate app installs silently like some kind of trojan virus in another application? This is absolutely unacceptable. This is the kind of thing that both CCleaner and Avast should be trying to stop, not colluding to make it happen!