Linux

To re-install grub manually (mostly for raid systems, if you want more than one drive to be bootable)

grub
grub> device (hd0) /dev/sda
grub> root (hd0,0)
grub> 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.
succeeded
 Running "install /boot/grub/stage1 (hd0) (hd0)1+15 p (hd0,0)/boot/grub/stage2 /boot/grub/menu.lst"... succeeded
Done.

grub> quit

Repeat above for each drive. Note the (hd0,0) refers to the boot partion.