In the Run box type "sfc /scannow" without the quotation marks and press "OK". This will start the System File Checker immediately.
At this point you may be asked for your windows installation disk if it was unable to find the files it needs on the hard drive. Once finished it will automatically close.
If the first method was unsuccessful i usually use Checkdisk. I usually have more success with process. To run checkdisk, go back to the run command and type "chkdsk c: /r" without the quotation marks.
The next screen will be a command prompt. Type in the letter y and reboot to start checkdisk.
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