If you have either Vista Business or Ultimate edition, follow this procedure:
How to downgrade:
Install a copy of Windows XP Professional with the product key that came with that copy, and then when you come to the activation screen -- which is near the end of the installation process -- select the "activate by phone" option rather than the online method. You'll likely end up talking with a live rep; tell him that you're downgrading from Vista to XP, and give him the Vista product key. The rep will walk you through the rest of the procedure.
Note: Windows Vista Home Premium and Vista Basic are not eligible for downgrade rights.