If the computer does have all the drivers to support XP, then you can go to the store or online and buy the windows XP software. You will have to format your computer, before you install XP onto the computer. Also be sure to backup any important data onto CD/DVD/Flash drives, as it will be lost.
How can i change windows vista to windows xp on my computer?
It isn't worth it. Now that Vista SP1 is out, your best option is to stay with Vista, it's much better.