There is no button to press - you need the XP media to reinstall the Operating System - If you do this you will lose any applications and device drivers that you have added to the machine over time and system updates as well.
What you propose is relatively easy if you have the media disk - it is often best, however, to fix any problems the system has rather than rebuild the OS - you give no hint about why you wish to go back to square one.
Good luck...
Peccavi, May 2010
