Reinstall Windows But Keep All Files

So my father asked to reinstall Windows 10 on his desktop. He has a job where he might need documents that were created years ago any moment, so it is important that all the files gets transferred to the new install.

The problem is, the computer is extremely slow and filled with junk programs and probably a ton of malware or whatever. Is there an easy way to reinstall windows 10, and only keep files (documents, pictures, etc.) and not anything else?