http://lifeofageekadmin.com/fixing-windows-2008-r2-activation-errors/

​Location is at C:\windows\system32\Slmgr.vbs
/ckms = clear all KMS servers in cache and set to auto discover
/skms :  = Specify a KMS host server
/ipk  = Enter in a new license key
/ato = Activate the new key
/dlv = Display license information
First open a command prompt and try to register a license key. This example is for Windows 2008 R2 Enterprise Edition.
C:\> Cd c:\windows\system32
C:\Windows\system32>cscript slmgr.vbs /ipk
C:\Windows\system32>cscript slmgr.vbs /ato
If you receive activation successful you are complete. If you receive a 0xC004F074 error do the following.
C:\> Cd c:\windows\system32
C:\Windows\system32>cscript slmgr.vbs /ckms
C:\Windows\system32>cscript slmgr.vbs /ipk
C:\Windows\system32>cscript slmgr.vbs /ato
If you receive activation successful you are complete. If you receive a 0xC004F074 error again or another error do the following.
C:\> Cd c:\windows\system32
C:\Windows\system32>cscript slmgr.vbs /rilc
Reboot
C:\> Cd c:\windows\system32
C:\Windows\system32>cscript slmgr.vbs /ckms
C:\Windows\system32>cscript slmgr.vbs /ipk
C:\Windows\system32>cscript slmgr.vbs /ato
More information can be found at Configuring KMS Clients