Microsoft Knowledge Base Article 306525 contains the following summary:

If you are logged on as an administrator, the Automatic Updates feature in Microsoft Windows XP notifies you when critical updates are available for your computer. You can also specify the schedule that Windows follows to install updates on your computer. This article describes how to configure the Automatic Updates features in Windows XP.

Note This article assumes that you are using the latest Automatic Updates feature. Automatic Updates is included with Windows XP Professional, Windows XP Home Edition, and Windows XP 64-Bit Edition, but the latest Automatic Updates feature is included with Windows XP Tablet PC Edition Service Pack 2 (SP2), Windows XP Media Center Edition SP2, and Windows XP Professional SP2, Windows XP Home Edition SP2, and Windows XP 64-Bit Edition SP2.

If you use Automatic Updates with Windows XP Professional, Windows XP Home Edition, and Windows XP 64-Bit Edition, the feature may have been automatically updated on your computer. If the options that are described in step 2 of the Turn On and Use Automatic Updates section are not available, install Windows XP SP2 to obtain the latest Automatic Updates feature. For additional information about how to obtain SP2, click the following article number to view the article in the Microsoft Knowledge Base:

322389 How to obtain the latest Windows XP service pack