Since the password files are in /etc they are only available once the second partition is loaded.
That is, they are not in /boot, so you can't see them when you load the SD card on a Windows system.
The way the I would fix it is to put the SD card into a Linux machine, mount the second partition, and edit the /etc/shadow file.
Delete the characters between the first and second colon for your username and then your password will be nothing.
This is probably a little tough for anyone not familiar with Linux, but then, those people wouldn't likely have a Linux system to do this with anyway.
Does anyone know of an easier way?
As a last resort, you can just re-image the SD card, but I assume you have stuff on it you don't want to lose.
- Ted B. Hale