Even CCleaner registry cleaner can break Windows from what I've read.
Of course you have to take some wrong actions.
Now who ever heard of a computer user taking wrong actions?

I never tested this, but just might someday, when I'm about to do an image restore. But others have done something close to it with bad results.
Uninstall some programs that call for reboot. Include some Win 7 changes that require reboot. But don't reboot. It can wait, right? I mean, there's an option. “No, I will restart my computer later.” No big deal. Besides, I want to play some Solitaire, and think about maybe uninstalling some other programs I don't want.
As I play a little Solitaire, feeling proud of "cleaning" my system, I remember CCleaner. Let's do that. So I clear off a few GB of unneeded files. Nice.
Hey, let's check the registry. Whoa, lets get rid of that stuff. Backup registry?
Nah. CCleaner is the best, and never causes problems. Delete that registry junk!
Now what did I forget? Oh, yeah, I'm supposed to reboot.
Hit restart, Win 7 shuts down, and tries to restart. Tries.
But registry entries needed by the unfinished uninstalls are missing, so Win 7 is busted. Maybe it can be fixed, maybe not.
But there's no doubt what busted it. CCleaner Registry Cleaner.
Sound far-fetched? Not to me. I read it on the internets.
The horror!