Try the following steps: http://www.vistax64.com/tutorials/102005-reset-user-account-password.html
If that doesn't work, boot the computer to Safe Mode. Then open a command prompt with elevated permissions and type the following: Net user administrator /active:yes
That will enable the "REAL" Administrator account. This is like the Root account on a Unix box: You will have full and unquestioned rights over the entire computer. Restart. From there, you can edit the existing user accounts as required.
To disable the "Real Admin" account again, open a command prompt and type: Net user administrator /active:no