Yesterday my notebook (which came with Windows 7) simply went to a boot loop unless I change to OS to Linux o_O. Since reinstalling Windows 7 messes the whole MBR, a GRUB restore is required to get the multiboot back to work. These are the steps to get it working again:

  1. Boot with a LiveCD and in places, click the original grub partition get it mounted.
  2. Open the terminal.
  3. Verify if it's the right partition $ mount | tail -1. Something like /dev/sda5 on /media/0d104aff-ec8c-44c8-b811-92b993823444 type ext4 (rw,nosuid,nodev,uhelper=devkit) will be returned.
  4. To make sure this is indeed the Ubuntu boot partition, run $ ls /media/0d104aff-ec8c-44c8-b811-92b993823444/boot
  5. To restore the grub type $ sudo grub-install --root-directory=/media/0d104aff-ec8c-44c8-b811-92b993823444 /dev/sda --recheck
  6. Reboot and enjoy!