Download XP SP3
You can now download XP SP3 though windows Update or here
Brief Description
This installation package is intended for IT professionals and developers downloading and installing on multiple computers on a network. If you're updating just one computer, please visit Windows Update.
http://www.microsoft.com/downloads/details.aspx?FamilyId=5B33B5A8-5E76-401F-BE08-1E1555D4F3D4&displaylang=en