I have a realy strange problem in Ubuntu 11.10. I have by accident my password set to empty. If I wan't to install software now the system asks for a password, but I haven't one anymore. If I enter my old password it sais it's wrong and if I leave it empty it also does. Now I wan't to set my password back, but the system asks for the old password before I can change it. That password doesn't exist, so I can't do anything anymore! Can someone help me please?
passwd from a terminal. It'll ask you for your old password, and then for your new one.
If it doesn't accept the old one there, run
sudo passwd <username> replacing
<username> with your username. That will make sure you can change it without the need for your old one, in the hope that it accepts the sudo password.
You will need to boot in to recovery mode (press shift key after BIOS post and keep it pressed until the
grub menu is shown) and drop to a root console. There you will be able to change the password for your current user.
Here you can find a very good example of what to do to reset your password.