Restoring Debian Lenny Grub

Boot the machine using the installation-media (in your case dvd),
On the boot menu
Choose advanced options
Choose rescue.
you`ll get the usual questions of installation (language, keyboard...) and some stuff will be loaded.
Select the appropriate options till you get the Shell
then you`ll be asked the device you wanna use as root file-system (where / of debian is installed ) and next what you wanna

do. Then enter the command grub, you will get the grub prompt


select the root filesystem mounted to '/'
then
grub
grub>setup (hd0)
grub>quit
exit

restart the machine and enjoy