When installing Ubuntu it is important to ensure that it is up-to-date. Although you might have downloaded the latest available package, there might be important updates. To ensure that your installation is up-to-date you can (1) select the option "Download updates while installing" during the setup or (2) update the repositories and install the latest updates after the installation.
To do the latter you can open a terminal (Ubuntu 12.04 Keyboard Shortcut: Ctrl + Alt + T) and copy-paste the following command:
sudo apt-get update && sudo apt-get upgrade