Try a Live-CD to boot from and mount your root-device (e.g. mount /dev/hda1 /mnt). Look up in (/mnt)/var/log/ the log files, most likely "messages". It will only show the modules that loaded and the last one with the error.
Post it, we may be of help then.
edit: some more information about your hardware may be helpful, too... GPU, which HDD is what channel&controller?, Installed Kubuntu on which hdd (as shown in windows management, e.g. "SATA 0/1/2/3") and what partition (primary, logical,..), Raid?, chipset mainboard are the most important.