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.
Windows Defender Offline is a free standalone, bootable malware and virus remover from Microsoft that performs an offline scan of an infected PC to remove viruses, rootkits and other advanced malware.
This tutorial will show you how to update and use the Windows Defender Offline Tool to create a 32-bit or 64-bit Windows Defender Offline bootable CD/DVD, USB flash drive, or ISO file on any computer to help you start an infected 32-bit or 64-bit PC and perform an offline scan at boot to help identify and remove rootkits and other malicious malware.
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Security Essentials can only be installed on Windows Vista and Windows 7, as on more recent Windows iterations it is superseded by Windows Defender.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.
Once you grab the setup file you need, just double click on it and follow the on-screen instructions to apply the latest definitions to Microsoft Security Essentials.