Do the following to restore all your system files back to default:

1. Press Control + Shift + Escape. The Task Manager will open.

2. Click File -> Run new task.

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3. Click Browse.

4. Type %windir%\system32 in the File name: textbox and press Enter.

5. Right-click cmd.exe and click Run as administrator.

6. Type sfc /scannow in the window that appears and press Enter.

7. Restart.