The second release candidate of Windows XP’s Service Pack #2 is now available. The people who are interested on the update must make a registry change on their computers. This can be made by downloading a small program at the Microsoft Download Center. Once a user runs the program, the update will be offered.
This release includes support of HD Audio according to the article at pcmag.com. The final release is expected to be in April of this year. The number of this release is designated as Build 5508 while the previous release was Build 3311.
Source: PCMAG

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The registry changes mentioned above are only necessary if you wish to obtain and install the SP3 RC2 via the Windows Auto-Update.
These registry changes are NOT necessary if you plan to download and use the standalone installer, which is always the recommended method when dealing with Beta software.
Thanks for clearing that up Jim. Is it worth the download in your view?