It's reasonably easy .... the easiest way is if you already have an XP install CD
my favourite is Ultra ISO
but there are others - Magic Iso - let's you create bootable CD with menus
in the boot section you have an option to extract the boot sector from the install CD - also copy WIN51, WIN51IP and win51ip.SP2 from the CD
open a new ISO - copy the 3 files into the root
load the boot file (you extracted earlier) .... and there you go.
trouble is ... it won't do anything!
look here
http://www.nu2.nu/pebuilder/
here
http://www.nliteos.com/nlite.html
and here
http://integrator.siginetsoftware.com/
for some useful pointers on what you can use them for