50 GB free isn't a whole lot but should be enough for Windows, I think the rule is that you want about 10-20% of your Hardrive space free for windows. If I were you I would do one of two things. You can probably try and clean up your hardrive, delete things you don't need, use CCleaner to clear out the cache files or various programs and Windows, and maybe run a defrag.
If your computer is several years old I would suggest a completely reformat and reinstall of windows. Its a pain since you will have to back everything up and start from scratch but you will be surprised how much fast Windows is when it's fresh. Windows 10 also has a feature called Reset which I believe you can keep your data and Windows will just reinstall itself...