Step 4: You will see file extraction dialog box for a while.
After the file extraction dialog box closes, open MSE, and click Update to verify that the virus and spyware definitions have been updated.
Reset the Microsoft antivirus update engine on your PC 1.
Open command prompt as administrator and run following commands one by one Now to try to Update MSE again, if problem persists there is no other way than installing definitions manually.
Step 1: First you must know whether Windows XP, Windows Vista, or Windows 7 is running in a 32-bit or a 64-bit operating environment.
You can find it easily by opening system properties (use Windows Pause/Break key to open system properties).
Now open MSE and click on Update tab and click on update button, if the problem exists go to next step. You can use use Fix it Tool for System Maintenance offered by Microsoft for that.
If the machine is not connected to the Internet, it will display “Microsoft Security Essentials wasn’t able to complete the virus and spyware definition updates.
Make sure your computer is connected to the Internet and try again” error.
Step 2: Next, download the offline definition updates for MSE from Microsoft: Download for 32-bit Download for 64-bit Just save the file on a USB stick so that you can connect the USB stick to the offline machine and then update MSE.
Step 3: If you are on Windows 7/Vista, right-click on the downloaded file and select Run as administrator.
Note that Microsoft Security Essentials is relevant to Windows 7 and earlier versions, as it was was replaced by Windows Defender in Windows 8.