I had an issue similar to this one. If you have a graphics card installed on your system then you will need to remove it completely and run the installation without the Graphics Card in there. Then you can put it back in after the installation. It is important to note that if you used WUBI to install your 12.04 then you shouldn't do a revert to 11.04 without first making a partition with your existing memory. Installing 11.04 alongside 12.04 and then use a partition manager to format the 12.04 partition. After that then you can use that partition manager to expand the volume on the 11.04 partition. I suggest that you use a boot able USB also as it seems to function better than a CD. Again the Graphics Card is likely the best route as it happened to me like that. It can't hurt anything so try it.