Insert the Ubuntu Live CD and reboot. Fire up your terminal. You can find it at Applications -> Accessories -> Terminal
You will get the grub-shell prompt
Now, you will have to find out in which partition Grub had been installed before.
grub> find /boot/grub/stage1
In my case it is 9. Please take a note of the digit that comes in your terminal.
Next setup that partition.
grub> root (hd0,9)
Note : Enter the number returned for you, here in my case it is 9.
Bingo! Your Bootloader is restored.
Alternate CD Method.
This is by far the easiest way to the restore the bootloader which was lost during a Windows re-install or due to whatever reason.
Step 1 : Download the Ubuntu Alternate install CD from here.
Step 2 : Burn the iso image and boot the CD.
Step 3 : Scroll down a little and select the Restore Bootloader from GRUB option.
Step 4 : Thats It. Done. Eject the CD and reboot. Your Bootloader is back.