Jose,

Well I hope you're not up to something that might get you into trouble, that said... here are some links you may find valuable.

This site here has presented technical information on the method used.
Licenturion - Fully Licensed GmbH, Rudower Chaussee 29, 12489 Berlin, Germany

How to change the Windows XP Product Activation Key Code

Unlocking WinXP's setupp.ini

How to change the Volume Licensing product key on a Windows XP SP1-based computer

XP Serial Change

Windows XP Prompts You to Re-activate After You Restore Your Computer

CD Key Reader

RockXP