How can i switch my OS?

Hi. I've bought an asus X50Z. It Came with vista home premium. i dont like vista so i want to switch to XP Professional. When i insert my original XP Professional CD, used once, on my HP Desktop Pc, It Worked.But, on the Asus, the Button that says " Install Windows XP Professional" is Unavailable. Can you help me switching my os? Thank you.