Unable to update Microsoft Security Essentials.

Install the latest Microsoft Security Essentials definition updates
http://go.microsoft.com/fwlink/?LinkID=87342
Step 1: Save or run the file. 
After you download the latest definition updates , click Run to install the definition file immediately, or click Save to save it to your computer. If you click Save, remember the name of the folder where you saved the file. 

Step 2: Install the definition update. 
To install the saved definition update file:
·       If your computer is running Windows Vista or Windows 7, click Start, click Documents, and then locate the folder where you saved the definition file. Right-click Mpam-fe.exe, click Run as administrator, and then click Run. When you are prompted for an administrator password or confirmation, type the password or provide confirmation, and then wait while the definition files are installed.
·       If your computer is running any other Windows operating system, you must be logged on as an administrator to install the latest definition file. Browse to the folder where you saved the file, and then double-click Mpam-fe.exe to install the latest definition file.
Also it Fixes any issues regarding the updates for Microsoft Security Essentials.