Utilities exist to back it up, restore it, and repair it. Heck, if I recall, there are command line utilities built into Windows to do a lot of that.
Yes, they do. But the ones included with Windows are very limited in what they can do, and all the third-party ones I'm aware of are both non-free and require a working Windows to run on — as, to the best of my knowledge, do the included Windows tools. So either way, if Windows has managed to piss in its registry badly enough that it won't boot, you're screwed, because if you can't boot, you can't run the tools to repair the registry that's stopping you from booting.
Third-party registry cleaner tools exist, too. I've tried several. Some are fairly reliable. But all of them warn up front that beyond a certain point, they're guessing when it comes to repairing bitrot or removing keys that they don't think anything owns any more, and that if it hoses your registry, you're on your own.
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Third-party registry cleaner tools exist, too. I've tried several. Some are fairly reliable. But all of them warn up front that beyond a certain point, they're guessing when it comes to repairing bitrot or removing keys that they don't think anything owns any more, and that if it hoses your registry, you're on your own.