Make sure you are signed in as main Administrator. If you already are signed in, does the Create new reset disk allow you to agree that the other disk will be unusable? There are more tips in windows 7 help about password reset disk and changing the password. 
I recently changed the log-on password for my dell 620 (windows 7 home premium), and attempted to create a password reset disk. A message box came up that said a reset disk already existed and creating a new one would make the old one unuseable. But it would not create a new one. How do I resolve this?
Password reset disk