I would assume you actually mean "downgrade from windows vista to windows XP"
in which case it may not be possible, depending on your computer there may not be drivers available for all your hardware that work with XP.
if you go to the manufacturers website you should be able to check if there are XP drivers listed for your computer, if there are then you simply buy a copy of windows XP and then wipe everything off your computer and install windows XP instead.
Though to be honest it is probably more bother than it's worth, just get used to using vista.
I saw this person on another thread earlier ^^^ and was waiting for the adverts to start. Westcliffe,please don't go to the overpriced and useless site advertised by anne.abraham.