The yellow shield in your system tray (bottom right hand corner of your screen) indicates that Microsoft has updates for your computer.

Mouse over the shield.      

 Click on the shield and open the Automatic Updates window.

You can just click Install and Windows will install all the updates it has for you or you can click Custom Install and Install and it will show you a list of all the updates it wants to install.

Once you have reviewed your updates, click Install

The Automatic Updates window will disappear and you can continue using your computer.

When it is done the system may need to be restarted. You can choose to Restart or just Close the window and restart later when it is more convenient.

 If you system has a red shield, Windows is not properly protected