If you know what partition the system is on it helps. You can actually use grub to help you find that.
So boot up into Linux and get to the grub tool with the command: grub
At the grub prompt use the following comands....
grub > find /sbin/init
That should get the available Linux partitions. For example, on my box .....
grub> find /sbin/init
Now, you use the partition for whichever distro you want to boot the grub menu from ....
grub> root (hd1,1)
Filesystem type is ext2fs, partition type 0x83
Now, you can tell grub to use the MBR with the command: setup (hd0)
grub> setup (hd0)
Checking if "/boot/grub/stage1" exists... yes
Checking if "/boot/grub/stage2" exists... yes
Checking if "/boot/grub/e2fs_stage1_5" exists... yes
Running "embed /boot/grub/e2fs_stage1_5 (hd0)"... 15 sectors are embedded.
Running "install /boot/grub/stage1 d (hd0) (hd0)1+15 p (hd1,1)/boot/grub/stage2 /boot/
Then exit the grub with this command: