The registry doesn't need to be cleaned. It takes up hardly any space on the system and one normally never looks at it, anyway.
I second that opinion.
If you :
- Don't know what the registry is or does.
- Don't know where the registry stores its data.
- Don't know how much space the registry takes up.
- Despite a multitude of other quicker, easier and faster means to find one, still find the need to ask on PPRuNe where to find a "free Windows registry cleaner"
You'll probably end up doing more harm than good tinkering with the registry.
Leave it alone.
There are many hundreds of other tweaks, hacks and clean-ups you can/should do to Windows before you even start contemplating messing with the registry.