Re: Downgrading VISTA to XP Pro
If you get the computer with XP factory-installed, the manufacturer will provide support for both XP Pro and Vista (Business or Ultimate) during the period covered by your support/maintenance agreement.
I would also advise against buying a computer with a low-end version of Vista and replacing it with your own copy of XP. There's no guarantee that your system will work, and you'll have no support from the manufacturer.