its best if you have the windows vista install cd. when you boot the computer with the windows vista cd it will detect the existing windows xp installation. it will give you options
1. if you want to install vista side-by-side with Windows xp -- creating a dual-boot option that allows you to select XP or Vista during computer startup
2. if you want to replace windows xp and upgrade it to windows vista
3. if you want to permanently delete the existing windows xp, and delete all programs and files on the computer. and then format everything, and run a fresh and new install of windows vista
also, depending on your motherboard manufacturer and model number, it might have a "recovery or reset to factory defaults" option. these is usually a last resort effort for resetting your computer to the original factory default settings. if this option is available for your computer brand and model, it typically deletes all the contents of your computer and places back the default OS and driver installation that was originally preconfigured when you bought the computer.
if the preloaded operating system is windows vista, AND you want to delete the existing windows xp installation and files and programs, then you may proceed with the reset to factory defaults option.