Welcome to LQ!!
(And thank you for asking a relatively easy question....
Boot into Kubuntu as you described, open a terminal and do as follows:
su <<to take on root powers>>
fdisk -l <<to confirm the device ID of your Linux partition. If you did a simple install, you'll have one partition mounted at / (aka root), and one for swap.
Let's assume that Linux is on hda1 (or maybe sda1). To GRUB, this is hd0,0>>
grub <<opens the GRUB shell>>
root (hd0,0) <<tells GRUB where to find it's files>>
Now we need to setup the GRUB config file (menu.lst)
note the name of the kernel file (vmlinuz--something**) and the initrd file**.
Make (or edit) an entry to read as follows:
kernel /boot/vmlinuz-something root=/dev/hda1 <<or sda1---see earlier>>
re-boot and everything should work.
** What I typically do is to alias these files so that I don't have to remember their exact names.
ln -s vmlinuz* vmlinuz
ln -s initrd* initrd
Post back if you get stuck somewhere----and/or look at the booting link in my sig. below.