i find it hard to believe that you installed windows 7 over xp without ever entering the existing bios password. the bios password is asked during every startup.
to remove a bios password, you must enter the existing password first. it won't work otherwise.
you could ask the owner of the pc.
discussing password cracking is against tomshardware's rules. http://www.tomshardware.com/forum/282333-31-read-first