SP3 will include all security updates and hot fixes previously made available as well as a series of new updates not least of which is the inclusion of Network Access Protection (NAP) to ‘help organizations that use Windows XP to take advantage of new features
in the Windows Server 2008 operating system’ as well as improvements in black hole router detection which will be enabled by default but, interestingly, Internet Explorer 7 will not be included.

XP SP 3 HERE