The problem seems to be in the Grub file.
If you installed Debian Lenny, you should have installed as default Grub 0.96 wich configuration file is in the path /boot/grub/menu.lst
But the problem is that you can't access any OS to edit this file. One way to solve is to use a live CD, to boot from the CD and edit that file from there.
You could use Knnopix, a debien based distro in live cd.
If you decide to go this way, from the console as root, do this:
This will show us the paritions info. After this we could give you a hand about how to edit your menu.lst file correctly (Grub configuration file).