Well intentioned "Registry Cleaners" can make mistakes that prevent rebooting.
Registry Cleaners downloaded from unknown sources may have bad intentions.
Registry Cleaners that are on-line and interactive would need full access to the entire computer before they can determine whether a registry key refers to an executable that does not exist (e.g was un-installed but left remainders in the registry).
I am sure that if you search the Internet hard enough you will find sites that will "fix" your registry while on-line and do NOT demand $$$ when you "click to complete repair";
but they may have already got your credit card details and financial records, and installed backdoors so that after they have "maxed out" your existing credit cards they can come back in to pick up any future cards ! ! !
If you want an exciting life full of technical dangers and financial risks this is the way to go ! ! !
Regards
Alan