You have to reinstall Windows.
In this case You need to format C Drive.
C Drive you cannot format whilst inside Windows OS.
Things to do before formatting C Drive.
- Copy desktop items and say place in folder fldrDesktop in another drive F.
- Copy My documents in a folder called fldrDocs_(your name).
- Copy all important files in folder backup_Win_10.4.21
- Move folders fldrDesktop and fldrDocs into backup_Win_10.4.21.
- Move to Google drive or Portable USB.