[SFSkol]

As far as Windows goes... I have used various utilities that clean the registry. I've customized the registry hundreds of times adding / deleting security entries, for large corporations. There always has been a controversy about the registry since XP. Microsoft doesn't want you fooling around with it. From personal experience on hundreds of PC's over the years that regular CCleaner of the registry does in fact do something. PC's run faster after cleaning. Leftover install / delete pointers must take up some space, registry files definitely get smaller. The drive properties does a pretty good job of getting rid of temp leftover files.

I once cloned a year old drive to an exact copy PC. Ran CCleaner registry fix. It loaded and ran faster. Go figure?