According to a Microsoft representative, <A HREF="http://www.infoworld.com/article/08/08/22/Will_Microsoft_still_activate_new_XP_installations-PC_World_1.html?source=rss&url=http://www.infoworld.com/article/08/08/22/Will_Microsoft_still_activate_new_XP_installations-PC_World_1.html">you will be able to activate new XP installations</A> for the foreseeable future. The fact that the company no longer sells XP "has no bearing on one's ability to activate XP installations..." Of course, the current rules about moving a Windows installation from one PC to another will still apply.
You can only do this with a retail version of Windows. The copy of XP that came with your computer stays with your computer. You must remove Windows from the old computer. Automatic activation will fail on the new computer. When that happens, call the toll-free number displayed on the screen and explain your situation to a human being. They will help you manually activate XP.