I just backed up my registry. That was eye-opening. It's 114 MB! That's huge. And yes, I have cleaned it with CCleaner (which is absolutely amazing). And I wonder why I don't switch to Linux. Hah. No registry to corrupt there.
Coincidentally, I remember a friend who had a really messed up registry. The computer would check it when it started up, and he had to quit the check application via CTRL-ALT-DEL. I eventually became a hero for fixing it (had to use a bit of DOS chops to get it done), but by then he'd gotten a new machine. I should ask him for his old laptop. Not a shabby machine to mess around with...but I digress.
Microsoft, why oh why did you make the registry in the first place?