Why do I need to reinstall Windows?
Think of going shopping. You see something you think you like, so you try it on. Then you decide you don't like it, and you put it back on the rack. You are absolutely no different than you were before you tried the item on. If you're shopping, that's the end of it.
In Windows, when you "try on" a piece of software and then you don't like it, you remove it. But unlike the clothes in the store, there's a good chance that your computer isn't exactly the same as it was before you "tried on" the software. Some small things were changed and not put back quite the way they were. It would be like a sweater you try on leaving "little bits of fuzz" on your clothes.
A couple of bits of fuzz is probably no big deal. They may be in places you don't even notice them. And there are even the software-equivalent of lint removers - they're called registry cleaners. They go through Windows and try to find those "bits of fuzz" and remove or repair them.
But sooner or later you reach the point where there is so much "fuzz" everywhere that it's just easier to throw out the old-and-dirty copy of Windows, and start with a brand-new fresh one. You won't put the same "fuzz" on the new copy, because you already know you don't like some other program, and you won't try it on again.
You can try to keep cleaning and cleaning the old copy, but after a while (usually a year or two) some spots are stained, and other spots are threadbare, and you're probably at the point that you'll be happier with a new, fresh install.
