Microsoft announced that,"Windows XP Service Pack 1 can be downloaded for free or ordered on CD beginning September 9th."

Windows XP Service Pack 1 (SP1) provides the latest security, reliability, and performance updates to the Windows XP family of operating systems. Windows XP SP1 is designed to ensure Windows XP platform compatibility with newly released software and hardware, and includes updates that resolve issues discovered by customers or by Microsoft's internal testing team.
hxxp://www.microsoft.com/windowsxp/pro/downloads/servicepacks/sp1/