bootable usb hard disk

sir my name is nirmal singh i m system administrator in goel institute of technology and mangement in this engnieering collage there is 120 pc and 60 lappys installing window xp in each machine is very complicated by CD so how i can make a usb hard disk bootable.i want to install windows xp operating system by usb hard disk .how i can make my usb hard disk bootable with windows xp setup files.please send my answer with all steps. thank u.