First (for future visitors), if you can't manage to boot into your system, pop in a Debian rescue CD or USB drive, mount your root partition at /mnt, and open a terminal and run
chroot /mnt
so that you can type commands in your installed system. If you have a separate /boot partition, type mount /boot to mount it.
You should have a file called /boot/grub/menu.lst. If you don't, run update-grub to create an initial file. The file contains “magic” comments, which the update-grub command transforms into actual Grub directives. Look for the following two lines (which begin with a #) and edit them if they don't match what worked for you:
# kopt=root=/dev/hda1 ro quiet # groot=(hd0,0)
Once you've edited these lines, run update-grub (again, if you had to run it once to create the file).
If you prefer to write menu.lst manually (which is only necessary in odd setups), here's how the section to boot Linux would look like:
title Debian GNU/Linux root (hd0,0) kernel /vmlinuz root=/dev/hda1 ro quiet initrd /initrd.img
update-grub(as root)? This should rebuild yourgrub.cfg.