SFC (System File Checker)

System File Checker gives an administrator the ability to scan all protected files to verify their versions. If System File Checker discovers that a protected file has been overwritten, it retrieves the correct version of the file from the cache folder (%Systemroot%\System32\Dllcache) or the Windows installation source files, and then replaces the incorrect file. System File Checker also checks and repopulates the cache folder. You must be logged on as an administrator or as a member of the Administrators group to run System File Checker. If the cache folder becomes damaged or unusable, you can use the sfc /scannow, the sfc /scanonce, or the sfc /scanboot commands to repair its contents.
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System File Checker Tool Syntax
Sfc [/Scannow] [/Scanonce] [/Scanboot] [/Revert] [/Purgecache] [/Cachesize=x]
/Scannow: Scans all protected system files immediately and replaces incorrect versions with correct Microsoft versions. This command may require access to the Windows installation source files.
/Scanonce: Scans all protected system files one time when you restart your computer. This command may require access to the Windows installation source files when you restart the computer. The SfcScan DWORD value is set to 2 in the following registry key when you run this command:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
/Scanboot: Scans all protected system files every time you start your computer. This command may require access to the Windows installation source files every time you start your computer. The SfcScan DWORD value is set to 1 in the following registry key when you run this command:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon
/Revert: Returns scan to the default setting (do not scan protected files when you start the computer).The default cache size is not reset when you run this command. This command is equivalent to the /Enable switch in Windows 2000.
/Purgecache: Purges the file cache and scans all protected system files immediately. This command may require access to the Windows installation source files.
/Cachesize=x: Sets the file cache size to x megabytes (MB). The default size of the cache is 50 MB. This command requires you to restart the computer, and then run the /purgecache command to adjust the size of the on-disk cache. This command sets the SfcQuota DWORD value to x in the following registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Winlogon