What speed did you burn Ubuntu at? You should use good quality CD-Rs or CD-RWs, burnt at below 8x speed.
The error message you describe sounds like some badly-burnt data on the disc.
When I try to boot ubuntu from a cd it gets through the part where it says Ubuntu and has the moving bar underneath, then the screen goes black for a while. Then the error message "Buffer I/O error on device sr0 logical block #########". Does anyone know what this error means and how to fix it? Thank You.