A Technical Preview of Windows XP Service Pack 2 is now available for download from Microsoft.com. You can find more information on this page. Keep in mind that this version of the service pack is available only as a preview for computer professionals who need to test software. You can find out more about how Windows XP SP2 affects developers by checking out the XPSP2 page we have on the Security Developer Center.

Here's the Warning from the download page:
WARNING!
This technical preview is unsupported and is intended for testing purposes only. Do not use in production environments.