Please help on vista computer?

I have a computer which had vista in it. I want to downgrade it to xp. I have the key, but after I set up the xp, the computer shut down by itself before I can get into windows. So now I think I should buy the vista key because even if I am able to install xp on my computer, it would not work properly anyway. I have a cd which called UPGRADE YOUR WINDOWS VISTA EXPERIENCE.which goes in the package when i bought my vista laptop. the question is if I want to upgrade it to vista ultimate, can I just put this cd in and install it normally or I have to buy the vista basic cd and then buy the key for that Upgrade cd? Thanks a lot for your help