It is the latter case when you need to download and install the Microsoft Security Essentials Definition Updates.
One important thing is to download the correct definitions for your system’s architecture: 32-bit or 64-bit.
Please note that Microsoft Security Essentials can only be installed on Windows Vista and Windows 7, as on more recent Windows iterations it is superseded by Windows Defender.
Therefore, you should point to the Windows Defender Definition Updates if you are running Windows 8 or higher.
Additionally, Microsoft Forefront Client Security ended its life cycle on July 14th 2015.
My freshly installed Windows Server 2012 installation with Forefront Endpoint Protection (FEP) installed just wouldn’t update.
Googling the error message gave me a lot of ‘you’re infected’ posts.