This sounds like it is most likely to be a broken CD. This could be that the CD is scratched or warped, burnt at too high a speed for the disk or drive, or even burnt from a broken ISO download.
There are some other possibilities; you might try looking at the menu that appears just before Ubuntu loads for options like "safe graphics mode" which fixes some problems. But loading initramfs is one of the first things that happens, and it only uses the CD and memory, so it's more likely to be the CD disk itself. At a stretch, it might also be the drive or even your RAM; have you used the drive recently?
You might try looking at http://www.ubuntu.com/GetUbuntu/download
for other ways of getting a working boot CD. Also try booting the CD on other machines if you can (you don't have to install it) to check the disk.
Hope that helps,
— Robert J. Lee