Do you have a copy of the Leopard Install DVD? If so, insert that in and then reboot the computer. While it's starting up, hold down the the C key and let the computer boot off the disc. There will be an option in the menu items to reset the password, which can then be done quite easily.
If you'd rather not do that or don't have the DVD, and don't mind a little single-user-fu, follow the commands
here exactly, and you can also reset the password that way. It's a little more difficult, but it works well, I've done it several times.