If you already have an open root shell, no problem, if not your only recourse will be to boot from CD, mount your root partition to /mnt, then vi /mnt/etc/shadow easiest thing to do then is leave the password field empty, and reassign a real root password IMMEDIATELY when you boot back up- you will be very vulnerable to attack for the 30 seconds or so between boot completion and the time it takes to key in a password, so best practice is to boot single user first and finish resetting the password- if you're really paranoid, yank out your network cables while you do it, and reconnect them before you exit the single user shell and come up to multi-user.
Vincent Esposito