It appears SP3 RC2 is available for Windows XP via Windows Update.

Previously only available to some thousands of beta testers a few weeks ago, Service Pack 3 adds several networking and security improvements, and compatibility with Windows Vista and Windows Server 2008 security credential providers. It also incorporates features previously available individually as separate updates, such as Micorsoft Windows Imaging Component (WIC), Microsoft Management Console (MMC) 3.0, and Wi-Fi Protected Access 2.

A list of all the features can be found on the Microsoft website.

Note: It is important to uninstall any previous versions of SP3 from your PC before installing this one.  This can be done from Add/Remove Programs in the Control Panel.

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