I recently downloaded ubuntu-12.04.3-desktop-amd64 from http://ubuntu.com.
I tried installing Ubuntu in two different ways:
I tried to install Ubuntu when my laptop was connected to the internet: It downloaded some packages during the installation. After the installation was finished, I couldn't boot to Ubuntu.
I tried to install Ubuntu when my laptop was NOT connected to the internet: An error message occurred when installing. The message was about "GRUB" and "booting".
My laptop is an Asus K46CB with a Core i7 processor.