in the grub console write this (note that you have autocompleet):
> root (hd,…)
> kernel /casper/vmlinuz file=preseed/kubuntu.seed boot=casper ramdisk_size=1048576 root=/dev/ram rw quiet splash
> initrd /casper/initrd.gz
> boot

peshka's reminder site – ver. 2.0
> root (hd,…)
> kernel /casper/vmlinuz file=preseed/kubuntu.seed boot=casper ramdisk_size=1048576 root=/dev/ram rw quiet splash
> initrd /casper/initrd.gz
> boot