Windows XP SP1 - End of support on October 10, 2006
Microsoft will be discontinuing update support for Windows XP SP1 and SP1a effective October 10, 2006. They will then only support Windows XP SP2 unless an extension is granted. It's my understanding that October may be the last update. Users on SP1 or SP1a may have only 2 months to move to SP2 (so that they are ready for the November updates).
http://msmvps.com/blogs/harrywaldron/archive/2006/09/14/Windows-XP-SP1-_2D00_-End-of-support-on-October-10_2C00_-2006.aspx