I use CCleaner, but I prefer using BleachBit( http://bleachbit.sourceforge.net/ ) because it seems faster, and there is a version for Linux, and it is open sourced as well.
There is a startup manager, Task Scheduler editor, web browser add-on disabler, registry cleaner, and other tools in CCleaner which may speed up your computer a little bit after you disable some startup programs, and scheduled tasks, but most of these tasks can be done by changing the settings in the programs like chat programs which startup during windows, or with MSconfig, Windows Task Manager.
Great program and can't get yourself into trouble with it, especially if you let it do what it recommends when dealing with the registry.
Major Geeks recommends it along with heap's of other reputable sites so that's got to say something!
I usually use it when I notice the computer is getting a bit overloaded with temp files - it does a good job of getting rid of the rubbish. Windows isn't anywhere near so good at cleaning up after itself and also, the index.dat files that have stuff about what you've done get removed on restart if you've asked Ccleaner to get rid of them - better privacy!
Some people use it weekly, some use it daily - it's a bit dependent on how you use your computer.