Reinstalling my Ubuntu box today (was a 12.04 – time to switch to 14.04) and the live usb key I tried (LUbuntu) couldn’t make it right away. Instead of the nice desktop, I got a completely garbled screen (and sometimes it doesn’t even get there and gives some errors about my graphic cards). The mouse was there, though and it looked like there was a window there that I could move, but that was all. I found out that it was a problem with the graphic card’s driver not accepting modeset in the kernel.
When booting from the live CD/USB, press Shift. A menu will appear with the language, and then offering to boot or install. Press F6 and select “nomodeset” in the list. Boot. 🙂