How to run a scannow check, if Windows is not loaded
Damaged system files is one of the common causes of incorrect operation of Windows. To restore them use a special utility staff SFC.exe running via command prompt running as administrator. SFC.exe can take several parameters, in particular /scannow when running a full scan of protected system files and restore them.
Usually check sfc /scannow is run from a running system, however, to implement this command when Windows can not boot. To do this you need to have installation disk with the system or written to disk the WinPE image.
Booting from the disk, hit Shift + F10. You will see a command prompt window. The next step will be to define the letters of the scanned section. This is necessary because the literal labels of partitions in boot environment differ from those used in a running system.
To do this, run in the command prompt, notepad, and after Notepad opens, select in its main menu «File» -> «Save as» and saving directory, specify «This computer».
In the Explorer window you will see the available partitions and their assigned letters. Most likely, the letter you have will be marked with a placeholder and the letter D, which is usually assigned to a custom tab will be marked in the system partition. That is to check sfc /scannow we will sections C and D.
Explorer and Notepad you can now close. Knowing the letters of the partitions that run the following command in console:
sfc /scannow /offbootdir=C:/ /offwindir=D:/
Will start system file checker, after which you are prompted to restart the computer.