Insert the Mac OS X Install disc and restart the computer.
When you hear the startup tone, press and hold down the C key until you see the spinning gear.
When the Language Chooser appears, select your language and click Continue.
Choose Utilities > Reset Password.
Follow the onscreen instructions to change the password.
Quit the Installer and restart your computer while holding down the mouse button to eject the disc.
You can change your login keychain password in Keychain Access to match your new account password to unlock your keychain when you log in.