i will try to explain my way with a short text.
You have to now how to start a computer from CD and u have to know how u install windows xp. For way one, u also have to know how to burn isoimages. And of course: you have to know how to find things in google 
If u only have one partition you have to divide your partition first in two partitions.
Way one: get a geparted cdimage and use this program to devide your partition. if u dont get a screen at bootup try the "force Vesa" option from the bootmenu.
geparted:
http://www.mikestar.com/Record/play;jsessionid=E6D28B38EA1D24D2DF3B8F4435AB1BF9?record=dZW6dG79jo25H
way two: u can use the vistabuildin partitioner. As i dont now the english names for the menus, i cant say were u found it in our version, but i think its controlpanel > administration > computeradministration > volumeadministration... there you can rightclick your partition to shrink it. But it often doesnt work.
next step:
Start your laptop from the windows xp cd. create a new partition and install winxp on this partitione (the second).
After that u have windows xp on your drive but vista doesnt start anymore.
Now u have to start your computer with the vistaDVD an click on repairoptions and that "fix startproblems" (as u now.. maybe this option has another name
).
After restart there must be an bootmenu with vista and xp. If it isnt use the program easyBCD to rebuild the bootmenu.
EasyBCD:
http://neosmart.net/dl.php?id=1http://neosmart.net/dl.php?id=1
If it still doesnt boot u have to use teh commandoline utility from teh vistaDVD. U will find a howto with google 
i hope it works for u, but of course, i dont take any warrenty 
hope my text is understandable 