Hope you've got your restore discs handy - you'll need them. (This is also why you NEVER carry around your original software discs with your computer.)
Have the machine already running, insert the first software restore disc, restart the machine and after the chime, hold down the C key to boot from the disc. Once machine has booted, select your language, then at the initial Install Mac OS X screen that appears, click on the 'Utilities' menu. Within, there should be a utility to reset your password.
Hope this helps, and welcome to BoS!