Windows has a number of design flaws, resulting in it becoming slower and slower and not lasting very long. You've probably heard more than once someone say "My computer is getting sluggish, I'm gonna reinstall". Reinstalling Windows solves the problem... until next time.
You may think this is just how computers work: they're very new technology, and not really stable yet. Well, try Linux and you'll be surprised. Five years from now, your system will be just as fast and responsive as the day you installed it, not to mention that you won't have any viruses, adware, trojans, worms, etc., that would force you to reinstall anyway.
I have managed to convince many people to switch to Linux, while keeping Windows on their hard disk, because they needed to use some piece of software that Linux doesn't have (eg Autocad), so they use both systems. Since the day they switched, most of them have reinstalled Windows about once in a year or two; but Linux didn't let them down, and is still running perfectly well and is still snappy today.
Linux lets you spend more time working, less time reinstalling over and over again.