GRUB MBR and restoring a partition

I tried to install another Linux O.S along with MS and to make two Linux's and one windows.Usually when I try this GRUB losses the ability to boot the first Linux install. It did it again. I would really like to get this first install bootable again.

Here are some facts.

-I have a rescue disk , a XP installation disk and a live Ubuntu Disk.

-If I use Gparted in the second Linux install; here by refered to as the second, I can mount the lost Distro but not of coarse boot it. From here I can get all the Grub files from this install.

-If I use the Rescue disk I can get the starting and ending sectors of the second install.

-The MBR is only 64 bytes long so this should not really be a burden.

So I need to either get the last installed O.S to chain load the lost one or get the MBR to point to the lost partitions boot sector.

Something I don't understand is how grub tranforms the MBR sector info to paths to find the partitions.

Any help will be appreciated!:wall: