Put your install DVD into the optical drive and reboot. As soon as you hear the boot chime, hold down the "c" key on your keyboard (or the Option key until the Install Disk shows up). That will force your MacBook to boot from the install DVD in the optical drive. When it does start up,you'll see a panel asking you to choose your language. Just press the Return key on your keyboard once. It will then present you with an Installation window. Completely ignore this window and click on Utilities in the top menu and scroll down to Reset Password.