If you can get an OS X CD, there is a utility that will allow you to reset passwords on it.
put the CD in the CD-ROM, restart the computer. you will need to hold the C key down from the moment the computer turns on until you get a progress bar, indicating that you are almost fully booted.
Once you are booted, look at the top of the screen for "Installer" or something to that effect. Click on it, and you should see something about resetting the passwords.
That should get you started. Good luck.