If Microsoft Security Essentials is not updating on your computer, there are a couple of things you can do to get it working.
Open up services on your computer (Go to search and type services.msc or go to Administrative Tools under the control panel and double click on services). Find Windows update and make sure it is set to Automatic.
Windows updates must be on. MSE will check for updates when definitions are greater than 24 hours old.
Verify your computer’s time is correct.
Also if you are a heavy user, go to settings and make sure the Start the Scheduled Scan is unchecked (when my PC is on but not in use).