I dont know if should help you in this issue, as it is windows 7 forums
Do you have the Xp disk ? Then boot from it, you will be able to install xp. During installation, format the partition where windows 7 was, and it would be deleted.
But pal , is it really necessary ? Can you please tell me how long you using 7? Is it really bad, or you are facing problems with it? Let us know what is your problem, we will try to solve it our best.
By the way, you have two alternative options to use xp and 7 side by side, virtualisation and dual booting. You may choose any of those. For virtualisation, you wil have all necessary information here : Virtualization - Windows 7 Forums
; and fo rdual booting , go here : Installation & Setup - Windows 7 Forums
Finally , hope you will be with 7