Windows® XP Service Pack 3 (SP3) includes all previously released updates for the operating system, in addition to a small number of new functionalities that will not significantly change customers’ experience with the operating system. This white paper summarizes what is new in Windows XP SP3, and how to deploy the service pack.
– Windows XP Service Pack 3 Overview [PDF]
April 14, 2008: Support is available for the release version of Service Pack 3 for Windows XP
April 21, 2008: Released to OEM and Technet/MSDN subscribers
April 29, 2008: Manual Update (Microsoft Update, Windows Update, Download Center), but was quickly removed when compatibility issues arose.
May 06, 2008: Re-released via manual update
June 10, 2008: Automatic Update (Windows Update)
Windows XP Service Pack 3 (SP3) build 5512 was released to manufacturing on April 21, 2008, and Microsoft planned to make it available to the general public on April 29, 2008 via Windows Update and the Microsoft Download Center. However, due to a compatibility issue with Microsoft Dynamics Retail Management System, the release of Windows XP Service Pack 3 was delayed.
The delay is because there is a “compatibility issue” between the XP SP3 and Microsoft Dynamics Retail Management System, a retail chain management program. Microsoft has now also removed the automatic distribution of Vista SP1 because of the same RMS bug issue.
Windows Service Pack Blocker Tool Kit 
A blocking tool is available for organizations that would like to temporarily prevent installation of Service Pack updates through Windows Update. This tool can be used with:
* Windows Server 2003 Service Pack 2 ( valid through March, 2008 )
* Windows XP Service Pack 3 ( valid for 12 months following general availability )
* Windows Vista Service Pack 1 ( valid for 12 months following general availability )
As we need to perform a large amount of testing against our standard operating environment, we may find that some packages are incompatible with SP3. In this case we may need to block the distribution of SP3 via Windows Update.
The Windows Service Pack Blocker Tool Kit is used to set or clear a specific registry key that is used to detect and block download of Service Packs from Windows Update.
Even if some sites are recommending that users should upgrade to SP3, use caution even for home systems as there may be gotcha’s lurking in the deployment that we are not yet aware of.
“Windows XP Pro SP3″ has now been added to our equipment register as a valid OS, but is official not supported by our unit or by our central IS area. Let the testing begin
 Windows Service Pack Blocker Tool Kit [MS]
 Windows XP Service Pack 3 FAQ [Paul Thurrott]
 Windows XP Service Pack 3 Resources for IT Professionals [MS Technet]
 Microsoft releases the long-anticipated Windows XP SP3 (updated) (2008-Apr-29) [ARS Technica]