From a current newsletter from MS.

End of support for Windows XP Service Pack 1
If you are running Windows XP Service Pack 1 or a previous version of Windows XP, you should upgrade to Service Pack 2. Starting October 10, 2006, Microsoft will end assisted support and security updates for Service Pack 1 as part of the life-cycle policy for all Microsoft software. If you currently use Windows XP, we recommend the free upgrade to Service Pack 2 to continue receiving the latest security updates.