You may be able to boot into maintenance mode (or something similar), but that may ask for the root passwd anyway!
Otherwise, you could boot from a "live" CD and then mount your hard disk to be able to get at the file as described in the first response (note that you delete the password field with the editor, not the whole line).
Once that's done, you can then reboot from your hard disk and login with no password.
Make a copy of that passwd file before you edit it, or you could get in a right pickle if you make a mistake.