Reinstalling OS X 10.6 without losing any data
I would strongly recommend against starting any recovery/reinstallation attempt without having a valid (and verified) backup available on another disk. As you are still able to boot the system with the Install DVD you can


use Disk Utility on the Install DVD to create a full backup copy of your internal disk to an external one
copy /Applications and /Users to an external drive (which should cover "all" critical data)


Then just proceed with an reinstallation attempt.