If you believe you're missing system files, a simple sfc /scannow from the elevated command prompt should suffice to fix it.
If you wish to be more thorough and really take a look at what's going on and fix any additional problems, (like bad sectors, etc.) then
from the elevated command prompt (start menu> cmd>right click and 'run as administrator') type: chkdsk /f /r and 'enter'.
You might well be told to shut the system down and the chkdsk will run on boot.
