I have dell xps m1330 vista OS.. and I want to change to xpsp2 . Recently I brought hp cd/ xpsp2 and even I tried a bootable xpsp2 cd, before installing I change my BIOS SETTINGS in onboard devices / sata operations to AHCI to ATA and I switched off flash cache module, and I saved and exit .but when ia trying to install from wizard vista is blocking by saying it can't accepts low version OS. So ,I request plzz provide full step wise info. So that I can get rid of evil vista... thanks

If you change to XP, there is the chance that some of the computers devices may not work properly as XP may not have all the drivers that your computer may need (ex.. usually this will affect network, modem and some other devices)

At this point, you would need to find compatible drivers for XP that will work in PC shipped with Vista.
-This would be trial and error to try to find compatible drivers that still may be needed once going to XP

Not saying it can't be done but possibility there will be some sort of driver issue

To install XP to laptop, I am sure you will need a version of XP that has SATA drivers integrated into the installation.
-More than likely the version you have purchased does not have SATA drivers and will not be able to install as the install disk will not recognize the hard drive

But you did mentioned the BIOS settings changes you have made and that may address the issue

This should help you with the install process
How to Format Drive and Install Windows from Scratch
Geek to Live: How to format your hard drive and install Windows XP from scratch - Feature - Lifehacker

I do not recommend downgrading a Vista equipped machine to XP, especially if it's a new machine (less than a year old). The likelihood that hardware will not work under XP is very high.

If you don't like Vista wait for Win 7.

Thanks very much for your response and your answer