How to restore default system files

This command might come handy for you if you are in trouble.

Make sure you know what you are doing.

C:\Windows\system32>sfc

Microsoft (R) Windows (R) Resource Checker Version 6.0
Copyright (C) Microsoft Corporation. All rights reserved.

Scans the integrity of all protected system files and replaces incorrect version
s with
correct Microsoft versions.

SFC [/SCANNOW] [/VERIFYONLY] [/SCANFILE=<file>] [/VERIFYFILE=<file>]
[/OFFWINDIR=<offline windows directory> /OFFBOOTDIR=<offline boot directory>
]

/SCANNOW Scans integrity of all protected system files and repairs files
with
problems when possible.
/VERIFYONLY Scans integrity of all protected system files. No repair operati
on is
performed.
/SCANFILE Scans integrity of the referenced file, repairs file if problems
are
identified. Specify full path <file>
/VERIFYFILE Verifies the integrity of the file with full path <file>. No re
pair
operation is performed.
/OFFBOOTDIR For offline repair specify the location of the offline boot dire
ctory
/OFFWINDIR For offline repair specify the location of the offline windows d
irectory

e.g.

sfc /SCANNOW
sfc /VERIFYFILE=c:\windows\system32\kernel32.dll
sfc /SCANFILE=d:\windows\system32\kernel32.dll /OFFBOOTDIR=d:\ /OFFWINDI
R=d:\windows
sfc /VERIFYONLY

C:\Windows\system32>