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Taken from microsoft website
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Schedule Automatic Updates

Note To modify Automatic Updates settings, you must be logged on as an administrator or a member of the Administrators group. If your computer is connected to a network, network policy settings may prevent you from completing this procedure.

Windows Server 2003
To configure a schedule for Automatic Updates:

In Control Panel, double-click System.

On the Automatic Updates tab, click Automatically download the updates, and install them on the schedule that I specify.

Click to select the day and time that you want to download and install updates.

When critical updates are detected, Automatic Updates automatically downloads these updates in the background while you are connected to the Internet. After the download is complete, Automatic Updates waits until the scheduled day and time to install the updates. On the scheduled day and time, all local users receive the following message that has a five minute countdown timer:

Windows is ready to begin installing the updates available for your computer.

Do you want Windows to install the updates now?

(Windows will restarts your computer if no action is taken within 5:00 minutes)

If you are logged on as an administrator, when you receive this message, you can either click Yes to install the updates or click No to have Automatic Updates install the updates at the next scheduled day and time. If you do not take any action in five minutes, Windows automatically installs the updates.

IMPORTANT: You may have to restart your computer to complete the update installation.

In Windows 2000

In Control Panel, double-click Automatic Updates.

Click Automatically download the updates, and install them on the schedule that I specify.

Click to select the day and time that you want to download and install updates.

When critical updates are detected, Automatic Updates automatically downloads these updates in the background while you are connected to the Internet. After the download is complete, Automatic Updates waits until the scheduled day and time to install the updates. On the scheduled day and time, all local users receive the following message that has a five minute countdown timer:

Windows is ready to begin installing the updates available for your computer.

Do you want Windows to install the updates now?

(Windows will restart your computer if no action is taken within 5:00 minutes)
If you are logged on as an administrator, when you receive this message, you can either click Yes to install the updates or click No to have Automatic Updates install the updates at the next scheduled day and time. If you do not take any action in five minutes, Windows automatically installs the updates.

IMPORTANT:
You may have to restart your computer to complete the update installation.