Use Bitdefender or Kaspersky free anti-virus instead.

Open a terminal as administrator ( right click the terminal (cmd) and choose open as administrator ):

type sfc /scannow

sfc stand for sys file check so it should fix your problem. If not try :

DISM /Online /Cleanup-image /Restorehealth

If it fails, backup your stuff and do a fresh Reinstall.

In Soviet Russia, PC break you