Finally Windows XP SP3 is available for the general public, after distribution was halted (Windows XP Service Pack 3 via Windows Update Delayed) when Microsoft discovered a compatibility glitch involving them and Microsoft Dynamics Retail Management System. Automatic distribution of Windows Vista SP1 is also resumed now. To help protect customers, a filter is put in place to prevent Windows Update from offering both service packs to systems running Microsoft Dynamics RMS. A fix is being tested and it will be made publicly available via Microsoft Download Center this month.(TechNet Forum)
Here are the links to XP Sp3 from Microsoft Download center:
IF YOU ARE UPDATING JUST ONE COMPUTER: A smaller, more appropriate download for individual users is now available on Windows Update. The best way to ensure you get Windows XP Service Pack 3 is by turning on the Automatic Updates feature in Windows XP. You can use our step-by-step instructions or, if you prefer, let us do it for you.
The Microsoft Download Center site is your only authorized web source for downloading a licensed copy of Windows XP Service Pack 3. To report a website offering unlicensed copies of Windows XP SP3 for download, please send e-mail to: firstname.lastname@example.org or visit http://www.microsoft.com/piracy/ReportingUs.mspx.
IE8 beta 1 and Windows XP SP3 : Also one more thing to Note is if you already have IE8 Beta 1 installed on XPSP2, Windows XP SP3 will not be offered to you via Windows Update. This is because after you update your system to XPSP3, you will no longer be able to uninstall IE8 Beta 1 and the Remove option will be grayed out under the Add/Remove programs in Control Panel. The reason is the same as in IE7 case described above. Since people are more likely to uninstall beta software, we strongly recommend uninstalling IE8 Beta 1 prior to upgrading to Windows XP SP3 to eliminate any deployment issues and install IE8 Beta 1 after XPSP3 is on your machine.
For more details check IE and Windows XP Service Pack 3
Also check Windows XP SP3 RTM Screenshots, when I had installed it last month when it was made available on MSDN/TechNet.