Ubuntu noob here, replacing Windows XP on a dell dimension 9200. Installed 14.04 from USB, and everything worked before/during install. Now after booting, if I don't choose advanced options, etc, the screen freezes(purple screen displaying Ubuntu, 5 dots below, and enter passphrase box). Keyboard and mouse work before this screen is displayed. Using a Logitech USB keyboard/mouse combo, and I believe an Nvidia graphics card.
In the terminal (press Ctrl+Alt+T to open or otherwise login in recovery mode, jump to root command line then try this) run this
sudo apt-get purge nvidia* sudo reboot
This will remove any installed nvidia drivers.
Then install the new drivers
sudo add-apt-repository ppa:graphics-drivers/ppa sudo apt-get update sudo apt-get install nvidia-361 sudo reboot
This will fix at least the nvidia issue - once you can login you can determine what other issues are there?