How do you delete vista?

I ahev had Vista for little over a year now and i'm sick and tired of its problems and just want to switch back to XP so i backed up all my work , I've got the XP CD and now I need to format my hardrive , but it won't let me ? I have also heard that you can have Vista and XP alongside each other ,so that you chose when you boot up , that would be ideal . Please help ? Thanks.