"To aid IT professionals in planning and testing for the deployment of Windows XP Service Pack 2 (SP2), Microsoft is making available this preview, based on Release Candidate 2 (RC2) of SP2. Additionally, we have established 11 newsgroups for sharing information."
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Windows XP SP2 RC2 Express Installer (via Windowsupdate).
Windows XP SP2 RC2 Network Installation.
View Release Notes and Information.
More here. Official Microsoft Technet article here.