Service Pack 3 has shipped to manufacturing for Microsoft Windows XP. Bits will be available for download and through Windows Update on April 29.
Users will be able to update Windows XP to SP3 at that time, but you will have to select the optional download in Windows Update or download a larger, stand-alone installer from the downloads site. Users won't be automatically updated early summer,
For more infomration, see Windows XP Service Pack 3 (SP3) Released to Manufacturing on TechNet.