This procedure gives you a full root shell! You can damage your system if you are not careful!
When booting up press SHIFT and use the arrow keys to select the rescue mode option and press enter.
The file system may be read only (it is in all currently supported releases). Remount as read write
mount -rw -o remount /
This will boot the system in rescue mode and you should arrive at a prompt that looks like this
To reset your password type this in
Press return, then you will be prompted to enter and confirm a new password.
Once you are done resetting your password you can than switch back to the normal GUI mode by putting this in
If if appears to boot normally, but you see a message that says:
Give root password for maintenance (or type Control-D to continue):
then you will need a different solution.
If the "Standard Way" does not work for you and you recieve the "Give root password for maintenance" message, you can recover your password using the following steps
Reboot your computer
Press SHIFT or ESC at the grub prompt (as earlier).
Select your image.
Highlight the line that begins kernel and press 'e' to edit
Go to the very end of the line, change the ro to rw and add init=/bin/bash
press enter, then press b to boot your system.
Your system will boot up to a passwordless root shell.
Type in passwd username
Set your password.
Type in reboot
If this doesn't work I am out of suggestions.
Correction, my only other suggestion would be a clean install, but this should work.