Installed 11.10, I tried to start 12.04 from CD but just got the purple screen with color changing dots and then name Ubuntu. CD drive control LED shows 2 seconds with 5 seconds pause... Drive constantly starts and stops. And, by the way, the hard disk Led indicates some data transfer, what, as far as I understand should not happen while starting ubuntu from CD
I ran into that in the past with ubuntu 10.10 on a couple of computers. The solution for me was to use the alternate installation cd instead. It worked like a champ.