Check your computer's documentation. You may need to go to the computer manufacturer's (OEM) website and look up your computer model number for the documentation. Most OEMs will have a way for you to make a Recovery DVD from the recovery partition on your computer. This way you will still have Vista for future use if you change your mind later.
You should be able to just boot from the XP installation DVD and format the hard drive, then install XP.
As Brink stated, ensure you know the hardware in your computer and get whichever drivers you need.
Once you do, boot off your XP install disc and follow the instructions. Plain and simple.
In the event you are using SATA hardrives, ensure you either get SATA drivers that you load during the XP install or, alternatively, disable anything to do with "SATA Native Mode" in your BIOS; otherwise the moment you reach the part where you choose which drive to install on (during the XP install) it will produce an error related to not recognizing any hardrives.
