You have thirty days to activate a new installation of Microsoft Windows XP before it stops working until you finally activated the installation. Activation works this way that you have to provide an installation number to Microsoft either by internet or by phone. If you change hardware or install the operating system again you will have to activate the product again as well. This does not make sense in my opinion but hey, they surely have an important reason for this procedure.
http://www.ghacks.net/2006/07/18/backup-or-restore-windows-xp-without-new-activation/
Kurbli 