Ok, what you want to do is use the Kubuntu live cd to fix this problem.
First, boot up into the live desktop.
I don't know how you partitioned your drive, but I'm assuming you following my suggestion of creating 3 partitions for root, home and swap.
We can walk through this and if you don't know which partition is root, we'll have to go back and forth a little bit to figure it out.
Open a terminal window from the live desktop.
type: cd /mnt
this will put you in the mnt directory.
now you need to create a directory to mount the root partition of your hard drive on.
type: sudo mkdir root
then type: ls
and make sure you can see the new directory called root.
I'm going to assume that the root partition of your hard drive is the FIRST partition created. If it isn't, this is where we'll have to make some adjustments.
Now to mount your root partition (if it's the first one), type: sudo mount /dev/hda1 /mnt/root
The above line makes a few assumptions.... one, that the hard drive you have problems with is the first hard drive in your system and that it is an ide hard drive. Based on comments you've made in the past, the ide part is probably right, but if it's not your first hard drive, you may need to use hdb instead of hda.
The '1' part is the partition number. If the root partition is the first partition of hard drive a, then the device is /dev/hda1 like we used above. If the root partition is second, then it would be /dev/hda2 and so on.
You can use the Partiition editor to determine which partition your root partition is, if you don't already know.
Ok... now provided everything up until this point worked, you should be able to type: cd /mnt/root
and you should see a directory similare to this:
bin cdrom etc initrd lib lib64 media opt root srv tmp var
boot dev home initrd.img lib32 lost+found mnt proc sbin sys usr vmlinuz
If you do, great! Let's keep going to get this thing fixed. If you don't, let me know and we'll figure it out.
Ok, so you have your root partition mounted and NOW we can edit the messed up boot menu.
Close the terminal window.
then type: kdesu kate /mnt/root/boot/grub/menu.lst
This will open kate with the CURRENT boot menu list, which is the one you said you made a mistake in. If you know which line you changed from hd0,0 to hd0,1 then you can just fix it and save the file again. If not, you could open your backup file, edit it according to my original directions and then resave it as menu.lst again. Just be sure not to overwrite the menu.lst.bu file you made.
Hopefully this will help you out. If you have any problems, let me know and we'll see what we can do to get it fixed. Remember... you don't have to reinstall just because you have a problem booting. You can ALMOST always fix it one way or another. The hard part is just knowing where to look and the steps to fix things are a tad bit different than they are in Windows.