Windows XP Service Pack 3 (XP SP3) will be available for download in Q2 2008 or about three yars after XP SP3 was released by Microsoft.
Paul reasons why Windows XP SP3 was delayed this long:
Microsoft was forced to temporarily halt development on Windows Vista in order to complete XP SP2. That’s because this release, though provided to customers for free as a typical service pack, was in fact a major OS upgrade and was developed outside of the company’s support structure, a first for any service pack release. After XP SP2 was completed, the people involved with that project moved onto other things, typically Vista or Windows Server 2008.
In the case of Windows XP SP3, Microsoft simply dedicated every available employee it could to completing Windows Vista, which by that time was years behind schedule. So it’s only been since the beginning of this year that anyone turned their attention back to XP’s next and neglected service pack. Link.