Start the computer in single user mode by holding down the (command + s) keys.

You will see white text on a black screen.

Don't forget to hit "enter" after each command.

You need to mount the system so type: sh /etc/rc

wait till it loads everything then type: rm /var/db/.AppleSetupDone

this will make the system launch the Setup Assistant when the computer restarts allowing you to create a new admin account for the computer.

To restart the computer type: reboot

Hope this helps.