repairing Windows OS files: SFC

Run the System File Checker tool (SFC.exe) as administrator.

1. Open an elevated command prompt. To do this, do the following as your appropriate:
   Windows 8.1 or Windows 8
   Swipe in from the right edge of the screen, and then tap Search. Or, if you are using a mouse, point to the lower-right corner of the screen, and then click Search. Type Command Prompt in the Search box, right-click Command Prompt, and then click Run as administrator. If you are prompted for an administrator password or for a confirmation, type the password, or click Allow.
   
   Windows 7 or Windows Vista
   To do this, click Start, type Command Prompt or cmd in the Search box, right-click Command Prompt, and then click Run as administrator. If you are prompted for an administrator password or for a confirmation, type the password, or click Allow.
   
2. At the command prompt, type the following command, and then press ENTER:
   sfc /scannow
   
   
   The sfc /scannow command will scan all protected system files, and replace corrupted files with a cached copy that is located in a compressed folder at %WinDir%\System32\dllcache. 
   The %WinDir% placeholder represents the Windows operating system folder. For example, C:\Windows.
   
   Note Do not close this Command Prompt window until the verification is 100% complete. The scan results will be shown after this process is finished.
3. After the process is finished, you may receive one of the following messages:
• Windows Resource Protection did not find any integrity violations.
  
  This means that you do not have any missing or corrupted system files.
• Windows Resource Protection could not perform the requested operation.
  
  To resolve this problem, perform the System File Checker scan in safe mode, and make sure that the PendingDeletes and PendingRenames folders exist under %WinDir%\WinSxS\Temp.
• Windows Resource Protection found corrupt files and successfully repaired them. Details are included in the CBS.Log %WinDir%\Logs\CBS\CBS.log.
  
  To view the detail information about the system file scan and restoration, go to How to view details of the System File Checker process.
• Windows Resource Protection found corrupt files but was unable to fix some of them. Details are included in the CBS.Log %WinDir%\Logs\CBS\CBS.log.
  
  To repair the corrupted files manually, view details of the System File Checker process to find the corrupted file, and thenmanually replace the corrupted file with a known good copy of the file.

From  Microsoft