Quite a few computers for sale come with XP and Vista is only an upgrade option. Look online for sites that let you build ur own machine to what ever spec you want, like here or here, they tend to have XP as a build option if you really dont want Vista.

Or you could go down the favoured route of building your own machine. That way you get to have exactly what you want and you will probably save a bit of money as well. I have just finished building the rig in my signature and will maybe do a step by step if people really want me to.