Update Microsoft Security Essentials definitions manually - no Windows Update required
You can't update your Security Essentials definitions automatically if the Windows Update service is disabled. And while keeping your system fully patched is a necessity, not everyone wants to allow Windows to do it for them - they prefer the going manual route.
Fortunately, Microsoft knows this. After all, Security Essentials is based on Forefront - their enterprise antivirus product. And in the enterprise it's common practice to install updates manually.
To that end, Security Essentials definitions can be downloaded as a standalone executable from Microsoft. They're available from the Malware Protection Center where you'll also find the current defs for Forefront and Defender.
Download the .exe yourself, run it, and your Security Essentials is up-to-date! Better yet, set up an app like Ketarin to handle scheduled downloads for you and save yourself a step.