I have Windows 7 64 on my laptop (MSI GX660). I downloaded the 64 bit Ubuntu 12.10 install onto a 1GB USB drive, rebooted, and installed onto a 1TB USB external HDD. I followed this guide
I then set the boot priority in the BIOS to the external. Once it gets to the purple menu where I can choose "Ubuntu", I do and it goes to a purple screen and stays there. Choosing Ubuntu recovery gives the same result. I've tried typing "e" to edit and changing certain lines to "nomodeset" and others, doesn't work.