That Toshiba will come with Vista pre-loaded. You have to install the recovery CD that comes with the machine to load XP Professional. It is very straightforward.
Incidentally they have a couple of quid in hand on that machine - you may be able to get them to shave off another £10.
Also Toshiba offer an extension to a 3 year collect and return warranty for £50 + VAT might be worth considering.