This has been asked a lot here.
Note there may not even BE drivers for XP for the components in this laptop.
You will need to a do complete reinstall of Windows as there is no path to downgrade.
You will obviously need a genuine Windows XP CD, and all the XP drivers, plus XP versions of all the programs you want to reinstall afterwards.
You will need to "boot" the Windows XP CD, go though the install process, format the hard drive (which will delete everything on the drive), install Windows XP, then install all the drivers, then install all the programs.
This is a LOT of work and will probably take you over 4 hours (even if it all works first time).
When I do an XP install it can take all day by the time I have done Windows updates, and installed all my programs, anti virus etc etc etc.
Unless you have a very good reason I would stick with Vista.