CCleaner is the best system cleaner for Windows I’ve used. It’s never damaged a system, and it fixed an issue I had once on Windows 7 where the Control Panel wouldn’t load.
Its two main uses are cleaning up your filesystem and cleaning out your registry. Some argue your registry doesn’t need cleaning, and in most cases that’s true. However, the registry cleaner fixed the Control Panel issue, so it’s clearly useful.
Here’s a quick demo.