Windows XP Service Pack 1a (SP1a) provides the latest security and reliability updates to the Windows XP family of operating systems, and includes Internet Explorer 6 SP1a. Windows XP SP1a 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. To determine whether to install Windows XP SP1a, we recommend reviewing the Windows XP documentation and information below. If you have already installed Windows XP SP1, you do not need to Windows XP SP1a.
For further information about the specific difference between Service Pack 1 and Service Pack 1a (SP1a) for Windows XP, see article 813926, Differences Between Windows XP SP1 and Windows XP SP1a.
http://support.microsoft.com/?kbid=813926