Microsoft is committed to providing quality products to customers.  As part of this commitment, we would like to remind you that Windows XP Service Pack 3 (SP3) will be released to Automatic Updates shortly.  The third service pack to Windows XP includes the previously released updates and hotfixes to Windows XP, creating a new baseline for servicing.  

Optional Actions

If you wish to prevent users from installing Windows XP SP3 through Automatic Updates, Microsoft recommends you take one or more of the following steps:

1. Download and deploy the Windows Service Pack Blocker Kit. The Blocker Toolkit is available in the Microsoft Download Center. 

2. Deploy an update management solution that provides full control over the updates you deploy to computers in your network (Compare Update Management Solutions).  IT Administrators using an update management solution should use their product’s standard features, rather than the Blocker Toolkit, to control SP1 distribution.

Source- MSDN Blog