Costumava haver um script vbs para os servidores 2008 que fez isso; no entanto, parece um novo método (da Microsoft) aqui que abrange o Windows 8 e o servidor 2012.
Run the following command at an elevated command prompt: Cscript.exe %windir%\system32\slmgr.vbs /ipk
Note You can also use the Volume Activation Management Tool (VAMT) 3.0 to change the product key remotely, or if you want to change the product key on multiple computers.
As instruções que usei para 2008 são:
Open the command prompt and navigate to %windir%\System32 directory
slmgr.vbs -cpky (this clears the KMS entry you may have)
slmgr.vbs -upk (this removes any product key installed)
slmgr.vbs -ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx (where the x’s are the new product key you want to use)
slmgr.vbs -ato (this activates the server)
Type slmgr.vbs for further options….