Accidentally if GRUB of your dual-boot system with Linux and Windows was deleted either due to your monthly reinstall of Windows, and now you don’t see the linux bootloader anymore, so you can’t boot into Ubuntu or whatever flavor of linux you prefer.This is the easy way to re-enable Grub.

  1. Boot off the LiveCD
  2. Type in the following commands in a Terminal and, noting that the first command will put you into the grub “prompt”, and the next 3 commands will be executed there. Also note that hd0,0 implies the first hard drive and the first partition on that drive, which is where you probably installed grub to during installation. If not, then adjust accordingly.
sudo grub
>root (hd0,0)
>setup (hd0)
>exit

Reboot (removing the livecd), and your boot menu should be back.

Only read below if Windows is now missing from the boot menu

If you installed Ubuntu before you installed Windows, then Ubuntu will not have anything in the grub configuration for Windows. This is where you’ll have to do a bit of manual editing to the grub boot menu file.

If you open the file /boot/grub/menu.lst with the following command:

sudo gedit /boot/grub/menu.lst

You’ll see a sample section for Windows, which you’ll want to uncomment and add to the boot menu list in whatever position you want it in. (uncomment by removing the #’s)

# title   Windows XP/Vista
# root   (hd0,0)
# makeactive
# chainloader   +1