i'm still running xp, but would it not just be a case of formatting your drive and installing xp? I wouldn't have thought you would be able to install xp over the top of vista (although i could be wrong).
Well, you probably won't have to format the target partition if you don't want to.
another option for them, (which i use myself) is a 'dual boot' option, i have 4 seperate hard drives (though you only need 2), and have vista as my main operating system and XP on the other drive for 'as and when i need it' (compatibility issues for example). That way, you get the best of both worlds, though i mainly use Vista now. It'll cost maybe £30/£40 for a new hard drive but it's extra storage too
another option for them, (which i use myself) is a 'dual boot' option, i have 4 seperate hard drives (though you only need 2), and have vista as my main operating system and XP on the other drive for 'as and when i need it' (compatibility issues for example). That way, you get the best of both worlds, though i mainly use Vista now. It'll cost maybe £30/£40 for a new hard drive but it's extra storage too