A recovery disk is not an "installable" version of Windows but an image made of the system drive after Windows, drivers and other required software was installed and configured. The recovery disk is used to boot the system and to automatically copy that image back to the system drive when the system is booted from it. You wont's even need to uninstall XP - the recovery process will simply overwrite it. Because of this you should back up any important data you stored on the drive. The recovery process will also often rest and changes made to the disk partitioning so if you have created any partitions on the drive you should also back up their contents first. It is important to realise that because the image is of a specific system the recovery may be run on that system only.