If you discovered that one or several system files are corrupted or deleted, there is a way to restore them from Windows installation CD, DVD or ISO image.Instructions for Windows 10, 8, 7 and Vista1. Download and install the latest version of 7-zip. You may need to download one of the betas, because the latest stable release of 7-zip might be too ancient to open WIM files from the newest versions of Windows.
2. Open the root of the installation media.
3. Open the Sources folder and find the file named install.wim.
4. Right-click the install.wim file, then point to 7-zip, and then click Open archive.
5 (optional). If you see one or more directories named from 1 to 6, then open the folder that has the highest number in its name.
6. Open the folder that contains the file you want to restore, right-click the necessary file and click Copy to.
7. When the Copy window appears select the folder on your hard drive where the file should be placed and click OK.

I've had this issue with a couple computers (my own!)  For some reason the Windows Update services is turned off.  To turn this service on:
​1. Press Win+R keys together to launch RUN dialog box. Type services.msc in the textbox and press Enter to open services manager.
​2. Scroll down to Windows Update and double click.  Change the Startup type to Automatic.
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​How to repair and reboot to Windows 7:a) Insert the Windows 7 System Repair DVD or Installation DVD and reboot.b) Select the correct keyboard input method and click ‘Next’.Note: If you are using the Windows7 Install DVD then at the 'Install Now' prompt (Do not pick the Install Now option) instead choose the 'Repair your computer' option located in the lower left of the display.c) The System Repair tool will search for a valid operating system and will report that “Windows found problems with your computer’s startup options. Do you want to apply repairs and restart your computer?”d) Click ‘Repair and restart’ and reboot again to the Windows 7 System Repair environment.e) Click ‘Next’ at the keyboard selection prompt and when the ‘System Recovery Options’ screen appears verify that the “Use recovery tools that can help fix problems starting Windows” is selected.f) Now from the menu list select the ‘Command Prompt’ option.g) At the DOS prompt type the following three commands:bootrec.exe /fixmbrbootrec.exe /fixboot (may return an 'Eliment not found' message)bootrec.exe /RebuildBcdh) Close the command prompt window and click the ‘Startup Repair’ option.i) The repair process may take some time, so wait until the process completes at which point you may see two messages “Windows cannot repair this computer automatically” and “Startup Repair cannot repair this computer automatically”.J) Close this message window by clicking on the X in the upper right corner of the Send/Don’t send window. Next click on the ‘View advanced options for system recovery and support’ option and from the main menu once again click ‘Startup Repair’. This time the repair process may only that a few seconds and when prompted “Startup Repair could not detect a problem”, click ‘Finish’ and run the ‘Startup Repair’ option one more time, click ‘Finish’, and then remove the System Repair DVD and click ‘Restart’. Reboot the computer.