Sorry I didn't get back to this one earlier. The screenshot shows it hanging on fsck, which is not unheard of, depending on the nature of the problem. I keep a bootable Parted Magic Live USB stick for such occasions -- if you fsck it that way, then *buntu will be happy to proceed with the boot, and then you can fix whatever is the problem. So now it is booting and letting you log in? If the only remaining issue is the appearance of the KDM greeter, that is controlled by System Settings > Login Screen.