The easiest way to create, or change the administrative password for a PC is to use a Low-Level access CD.
You will be booting from a CD that grabs the information of the operating system installed on the computer.
The program that I've used before in this situation is called ERD Commander. And you just have to download the .iso file and burn it to a CD and then boot from it.
You can do a search for ERD Commander or you can download from the following URL:
drsgeorgia.com/index.php/downloads/secti...ow-level-access.html
It has a selection in there called Locksmith which allows you to change the password of an existing user.
After you change the password, reboot and use the new password that you set up.