To disable Windows 7 / Vista from downloading automatic updates, follow these simple steps.

Open Start menu, right-click on Computer and click Properties. This will open System dialog box 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. You will now see a window that will look like this,

You will notice that Windows has enabled option as default, to disable it, simply select Never check for updates and you are done.

Note: Microsoft recommends that you must keep your windows up-to-date in order to remain secure. And I recommend the same, my computer is always up-to-date, this is one reason I never get attacked by any kind of virus.