When you log into Windows Server, the Server Manager opens automatically, but I rarely want to use it. In Windows Server 2008 R2 there was a checkbox on the main window which would allow you to dismiss the window and prevent it appearing again.
This option still exists in the Windows Server 2012 Server Manager, but it is no longer front and centre.
To prevent Server Manager loading automatically when logging into Windows, click Manage and then Server Manager Properties:
Mark the Do not start Server Manager automatically at logon and click OK to save the change: