Downgrade OS from Win 7 to Win XP

I bought an HP Pavilion P6-2011 Desktop PC for my spouse. It comes with Windows 7.
But we are more familiar with using Windows XP so we want to downgrade OS from Windows 7 to Windows XP Professional.
I tried every possible way to do it. But the computer won’t let me.
Please cite step-by-step method on how to do it correctly.
Thank you for giving me your valuable time.
