Uninstall the Windows Vista Upgrade
Windows Vista does not include an uninstall feature, making the removal of Windows Vista a manual procedure that is not available in all configurations. The ability to perform a manual uninstall using the Windows.OLD folder depends on how Windows Vista is installed. In some situations, only a clean install of the previous operating system returns a computer to that operating system.
Due to the complexity and time required, it is not recommended to uninstall Windows Vista. If you wish to have Windows XP back on the computer, it should be installed from the Windows XP backup media.