If you do a clean install, you will lose everything on the C drive.
You should back up ALL personal data on that drive to an external. I don't know if you keep your personal data on C or not. Most people do, usually in sub-folders of C:\Users\Your user name whatever it is.
Don't forget email and bookmarks. They may be somewhere else.
It's up to you to find out where your personal data is--people have different habits. If you type up a spaghetti sauce recipe, where do you put it?
Locate your Product Keys for any software you may use. Locate software discs.