As mikef said, you can only do this if you have a Vista Business or Ultimate licence.
If you do, install Windows XP Pro using the XP serial number, choose to activate via telephone.
Call Microsoft on 0801 43 43 43, tell the representative you're downgrading from Vista Business/Ultimate to XP Professional. They should ask you for the Vista CD key and the installation ID generated by the activation process.
Once they confirm the Vista key and install ID, they'll read out the confirmation ID over the phone, enter it and your XP install should be activated.