When I tried to upgrade 12.04 on my old 512 meg RAM machine to 14.04 it seemed to have trouble and now the system is unstable (screen title bar and application bar not appearing, etc.). I have downloaded a DVD with the .iso file for 14.04 on it, but my machine does not want to boot from the DVD drive. Machine is an HP 'Media Centre PC 873n' with 512 meg of memory. Any idea how I can get it to boot from the DVD drive? When I first installed Ubuntu it was a Windows PC and it didn't have any trouble booting the 12.04 Ubuntu from the DVD drive.