first you need to jump the laptop back into AHCI compatibility mode from sata, otherwise windows installation will fail.
first I advise you read up on creating a slip stream disk, using nlite, it will mean that if you want to go back from AHCI mode to SATA, you need to find 2 things
1. storage manager drivers from INTEL
2. the correct chipset for this MB
you create the nlite disk which you will incorporate the 2 above into a stage for driver insertion
now, here are the usual steps after all that
1. locate how to boot into bios. boot into bios
2. in the bios, look for AHCI mode and SATA, change the option to AHCI compatibility
3. make sure your xp disk is in the machine
4. start installing
after installation is complete, you are going to need to install a few basic drivers :
1. graphics card driver (go to http://www.omegadrivers.net/,
they have the modified inf graphic card drivers, best available)
2. officialized windows xp disk, so you can auto update through windows update
3. network card drivers, this is quite important
4. start to update your windows installation by using microsoft update through explorer
5. usually the update should find all the remaining drivers after a certain point.
get to that stage, and the rest should be plain sailing, you may bump into problems with the audio drivers, but do a search for those and you should be lucky, sometimes its important to find the model of your card and other components before wiping vista from the machine, so you can track the sp drivers down if they fail.
i would also advise you to check dell website for the XP drivers, sometimes they have them, sometimes not.
hope this helps