With what we sell here at our store, which is the OEM version ( we build our own commuters ) xp home and xp pro and xp 64 come on separate cds and you need a matching product key. But your product key ( for xp pro ) will work with any xp pro cd. With Vista, Win 7, and Office 2007 all the products are on the same DVD and depending on the product key you enter which version is installed. Your Win 7 product key will work with any Win 7 dvd ( even a copy from someone else, if you loose your DVD, or did not get one with your system ) but you will always get the product that the Key number calls for - home, pro - 32 or 64 versions.
Some companies may not include DVD and may charge extra for it.
Bill