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.
To do so, first you need to know your environment. Type msinfo32 in start search and hit Enter. Locate your architecture type next to the System Type line. If you see x86 then its a 32-bit PC. If you see x64, then you have a 64-bit PC.
If you visit the Malware Protection Center, you will be able to see the latest definition files available for Forefront, MSE & Defender.
Download the Microsoft Security Essentials virus and spyware definition update file that is appropriate for your version of Windows, as follows:
To download the file for a 32-bit (x86-based) version of Windows, go here.
To download the file for a 64-bit version of Windows, go here.
Right-click the saved mpam-fe.exe (or mpam-fex64.exe) file, and then click Run As Administrator. If you are prompted for an administrator password or for confirmation, type the password or click Continue.
When the definition update file runs, a file extraction dialog box appears. The dialog box indicates that the definition update is installing.
After the file extraction dialog box closes, you can verify that the virus and spyware definitions have been updated. To do this, open Microsoft Security Essentials, click Update, and then examine the virus and spyware definitions status. Sourced from KB971606.
How to update MSE without using Windows Update may also interest you.