In case you need to extend a Windows Server 2012 Core Installation with a desktop GUI do the following:

start a powershell
Get-WindowsFeature server-gui*
Install-WindowsFeature Server-Gui-Shell,Server-Gui-Mgmt-Infra
2013-04-24_1745

To remove apply: Remove-WindowsFeature Server-Gui-Shell,Server-Gui-Mgmt-Infra