Admin password/Formating

I was trying to re-format my sisters computer for her and i boot up using the windows xp cd and i hit R and get to the admin password prompt dialog. This is where i run in to trouble. She has no idea what it is. I’ve gone in to windows and changed every password to a simple easy one, even the adminstrator account in windows, i deleted her user account so i’m left with guest and admin. Yet i get back to this and am stuck. I’ve tried about every “default” password i could think of, even a blank password, yet none seem to work. Any ideas? if anyone has any ideas about how to get around and just skip this to go straight to reformating that would help to.