Install the latest Microsoft Security Essentials definition updates

Install the latest Microsoft Security Essentials definition updates for computers running x86-based versions of Windows

For more information please review the Knowledge Base Article: How to manually download the latest definition updates for Microsof...

If you encounter problems while trying to install definition updates for Microsoft Security Essentials, you can download and install the latest Microsoft Security Essentials definition updates for computers running a 32-bit (x86-based) version of Windows.

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 Yes. 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.

Note: If you do not have an administrator account on the computer, ask an administrator to log on and install the definitions file for you.

If you are running Microsoft Security Essentials build 2.0.0375.0 or higher, please also execute the following steps

Step 3: Save or run the Network Inspection System definitions for Microsoft Security Essentials After you download the latest definition updates , click Run to install the definition immediately, or click Save to save it to your computer. If you click Save, remember the name of the folder where you save the file.

Step 4: Install the definition update. To install the saved definition update file:

• If your computer is running Windows Vista or Windows 7, browse to the folder where you saved the definition file. Right-click nis_full.exe, click Run as administrator, and then click Yes. 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 nis_full.exe to install the latest definition file.

Note: If you do not have an administrator account on the computer, ask an administrator to log on and install the definitions file for you.

For more information please review the Knowledge Base Article: How to manually download the latest definition updates for Microsof...

Install the latest Microsoft Security Essentials definition updates for computers running x64-based versions of Windows

Download and install definition updates for Microsoft Security Essentials on computers running x64-based versions of Windows.

If you are running Microsoft Security Essentials build 2.0.0375.0 or higher, download and install the Network Inspection System definition updates for Network Inspection System for computers running x64-based versions of Windows.

Note: You must be running an x64-based version of Windows to run an x64-based version of Microsoft Security Essentials. For more information, please visit the Microsoft Security Essentials website. 

Ref:  http://www.microsoft.com/security/portal/Definitions/HowToMSE.aspx