There's never been a single case documented here of CCleaner Registry cleaner harming any PC. It may be the most respected single tool in the entire tech world. That said, i only use it to clean up after a new install and then maybe monthly at most. Use the Run Cleaner button then on Registry tab Check for and Fix Issues.
I run Puran Boot Time defragger afterwards, which as Bob mentioned gets at System files that are otherwise running and can't be defragged when defragging within the OS. The Intelligent Optimizer on Additional Operations tab will move most-used programs and files to the outer edge of the HD for faster reads. It's quite brilliant.