I recently purchased a used computer, and the owner could not remember the administrater password. It has 13.04 on it.
It may be possible to revover a lost administrator password as outlined in this question:
However in your case you may not want to keep data or applications from the previous owner. In fact it may even be a security risk you will be faced when settings were made which possibly could give another person unlimited access to your machine.
Therefore the recommended and by far the easiest approach to your issue would be to just proceed with a fresh install of Ubuntu: