I've used Ccleaner often, been using it for a few years but never had to buy it. (must admit, I haven't used it in a while now). But, I've only ever used the 'cleaner' section, and only on the windows tab, not the applications tab and I haven't used the registry section at all. I have a paid program that I used for registry clean, that was highly recommended by a good IT forum. It hasn't caused any problems. Ccleaner was also recommended to me by the same forum but only in the way I've described.