Microsoft released XP Service Pack 3 yesterday but it was quickly pulled back and delayed. It seems like you can download XP SP3 from the Windows XP Service Pack 3 Overview page. Apparently it was delayed because of a glitch in the way it works with MS Retail Management Systems (RMS).
Chris Keroack announced in the Microsoft forums :
In the last few days, we have uncovered a compatibility issue between Microsoft Dynamics Retail Management System (RMS) and both 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 Service Pack 3 (SP3) to the web.
To help protect 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 the web.
We are also testing a fix, and will make it available once that process is complete. Once they have installed the fix, Microsoft Dynamics RMS customers should be able to run both service packs.
Until then, 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.
At the moment I'm using Vista on my main computer but I'm planning on putting XP on the laptop I get in the next month fro travelling so it's good to hear the best version of Windows just got better :)