What operating system(s) do you currently have installed?
Are you able to boot any installed system? or only from CD/DVD?
How are you running update-grub? Just running update-grub from the CD won't work. Take a look at the link below, particularly the methods in Section 12 on Reinstalling Grub2:
If you have problems with any of the methods in the link you try, make note of them and any specific errors and post what exactly you did so someone can give specific help. Also, if you get Kubuntu working and put its Grub2 bootloader in the master boot record, when you subsequently install other operating systems, put the bootloader on the root partition of the new system and reboot Kubuntu and run the sudo update-grub command.