I'm assuming that you are referring to the master password for the FileVault encryption system. If so, and you can't remember either the user's password or the master password (if you assigned one), you are SOL; the only way to decrypt the drive image would be to brute force the password using something like vilefault.
If you are not using FileVault and just need to reset the user's login, the options should be under one of the menus once you boot off the Tiger install CD (IIRC, the menu is called Tools or Applications).