Here is a decent article for setting up a dual boot.
Adding a hard drive from another machine is a bit trickier.
You’ll need to remove all system drivers, video, sound, etc. then boot off your XP disc and do a repair.
With that completed, you simply reinstall all the system drivers.
Of course, this assumes you have a bootable XP installation CD.
As it’s a Dell, chances are you only have a recovery CD.