Here is ZE Rescue Disk that you should keep close to your computer !
- runs automatically Boot-Repair rescue tool at start-up
- light LXDE environment in order to repair even old PCs with little RAM
Download: official website (md5 is on this page). Then burn it on CD or put it on USB key via Unetbootin.
1) Insert the Boot-Repair-Disk and boot the PC
2) Select "Recommended repair" --> solves the majority of bootsector/GRUB/MBR problems
Official website of Boot-Repair-Disk