So I'm not going to re-invent this post. A recent article on How-to-Geek has explained this concept far better than I could.
Here's the article:
http://www.howtogeek.com/162683/pc-cleaning-apps-are-a-scam-heres-why-and-how-to-speed-up-your-pc/?utm_source=newsletter&utm_medium=email&utm_campaign=080513
So there ya go. Those paid apps are next to worthless, if not outright malicious, and do next to nothing for your computer. I've used CCleaner for years in my PCs, my clients' PCs, and have had no issues. And CCleaner's registry cleaner includes a backup option (which is on, by default), so you can undo any registry deletions you may perform, just in case the system becomes unstable.
