[astyanax]

Here is a decent article for setting up a dual boot.
http://www.sevenforums.com/tutorials/8057-dual-boot-installation-windows-7-xp.html
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.

[PieterCasparzen]

Adding a hard drive from another machine is a bit trickier.

Dual booting with 2 physical drives sitting in the same PC, is I think what he's talking about. I've done it with CentOS and XP. one physical drive is XP, one physical drive is CentOS.

At startup, the PCs BIOS prompts you for which physical drive you want to boot from.

Whichever drive you booted from, that's the O/S that's running. The other drive just sat there spinning, uselessly. XP could not understand the CentOS drive. CentOS could have enabled me to do stuff with the XP drive but I did not want to.

[Boogieman]

Luckily, you usually can make a bootable installation CD from a machine where XP is already loaded:

http://www.howtohaven.com/system/createwindowssetupdisk.shtml