Microsoft failed to deliver Windows XP SP3 due to a compatibility issue with the Microsoft Dynamics Retail Management System (RMS):
I wanted to update you on today’s planned release to web (RTW) (of XP SP3). In the last few days, we have uncovered a compatibility issue between Microsoft Dynamics Retail Management System (RMS) and Windows XP SP3 and Windows Vista Service Pack 1 (SP1).
In order to make sure customers have the best possible experience we have decided to delay releasing Windows XP SP3 to Windows Update and Microsoft Download Center.
To help protect our customers, we plan to put filtering in place shortly to prevent Windows Update from offering both service packs to systems running Microsoft Dynamics RMS. Once filtering is in place, we expect to release Windows XP SP3 to Windows Update and Download Center.
Until we have published a fix for this issue, we advise Microsoft Dynamics RMS customers to not install either service pack. Microsoft Dynamics RMS customers running Windows XP SP3 or Windows Vista SP1 should contact Microsoft Customer Support Services for additional information.
The fix is currently in testing and will be available as soon as that process is complete.
DailyTech reports the glitch also affects Windows Vista SP1 but for some reason Microsoft didn't decide to pull that release, they'll just halt the automatic downloads.