From the Desktop -
Click on the Start Menu
Click on Control Panel
Click on Automatic Updates
Confirm that the Automatic (recommended) option is selected
Windows Vista / Windows 7
Open Start menu,
Right-click on Computer and click Properties.
This will open System that will show basic information about your computer.
On the left sidebar, under See also heading, you will find Windows Update.
Clicking it will bring you to the Windows Update dialog box, now on the left sidebar click on Change Settings.
Confirm that the Install updates Automatically (recommended) option is selected