11.10 - Change user password while password is empty

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?

Try running 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 replacing 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.