please help to uninstall FreeBSD!!!

[Raisa Brokhshtut]

Hello,
 
My old desktop has FreeBSD that I have never used. One of the friends of my son installed it long ago, but no one used that PC since then. Now I want to get rid of this program and to install Windows. Every time when I boot this PC it prompts for a user login which I don't know. This guy who intalled FreeBSD is not around anymore.
 
Anyway, I would greatly appreciate if you would guide me how to uninstall that program. I don't have windows reskue cd. So I want to completly remove that FreeBSD from my PC and to install the Windows operating system from CD. 
 
Thank you
 
Raisa