After some delays Microsoft released the Windows XP SP3 on Windows Update for everybody.

I just tried several times to download the 66.9MB of the Service Pack 3, but it stalled on me every time.

Microsoft plans to automatically distribute XP SP3 in early summer. So you get it then, if you do not download it yourself earlier.

The compatibility issue between Microsoft Dynamics Retail Management System (RMS) and both Windows XP SP3 and Windows Vista Service Pack 1 (SP1) are not gone yet.
Microsoft only deployed a filter on Windows Update so systems running Microsoft Dynamics RMS cannot download the Service Packs.
Microsoft plans to have the RMS fix ready this month and will publish it on Microsoft Download Center.
Via this post on the Microsoft forums.