CleanMyPC is designed to help you clean your Windows registry, which is useful when your computer seems to be running slower than usual. In addition to this, CleanMyPC includes a lot of other tools that will help you optimize the performance of your system.
From my experience with it, this program proved to be a reliable solution for increasing the speed of my computer. First off, it uses an incredibly small amount of system resources: it works well with only 32 MB of RAM and 10 MB of free disk space. CleanMyPC offers a lot of scanning options, such as: registry integrity, ActiveX entries, uninstall entries, shared DLL files, file extensions, software settings, and many others. It should be mentioned that if you select all of them, the scanning process takes a lot of time. Also, the program returns a large list of found errors but it doesn't fix them until you buy its full version. In addition to the included registry tools, CleanMyPC also offers features like: privacy eraser, startup organizer, etc.
As a conclusion, a lot of similar tools are available out there, and before buying any of them, make sure that your system is indeed working slowly. Also, try to make sure that the errors they "find" are indeed real.