XP will no longer be available in stores or as a preinstalled OS after that date. I'm certain as with all the older OS's you will still be able to find some available. But it will be gone and have yielded to Vista. Not a thrill to me personally. The super low cost line of PC's will still have it for two more years. The models such as the Asus Eee PC and Intel Classmate PC will have XP Home.
System builders or small companies that build PC's from scratch can still buy until Jan. 31, 2009. Product support, including security updates, for mainstream users of Windows XP continues until April 2009.