If the computer came with Vista, its most likely best to just keep Vista. Many of your newer devices are not completely XP Compatible, and therefor, may have adverse consequences.
Still, if you must, you need your original XP disc.
You will not be able to save your data unless you have a place to back it up to. Any data that you do not back up will indefinitely be lost, and there is no chance of recovery. There is no way to uninstall Vista, you can only completely remove it along with all the contents of the hard drive and put XP or another operating system on there in place of it.
