In this tutorial you'll learn how to change or reset the Mac admin password without the installation disk. This solution implies the the creation a new account on the Mac OS X!
Procedure:
The solution is to create a new admin by deleting a specific file. You need to enter terminal and create a brand new admin account:
1. Reboot
2. Hold Apple +S after you hear the chime (Command + S on new models).
3. When you get prompted, enter these commands in order to create a new account (hitting return after each line):
mount -uw /
rm /var/db/.AppleSetupDone
shutdown -h now
4. After your Mac rebooted you should have a brand new account as administrator.