Boot-Repair-Disk is a free boot utility that can fix MBR and other boot sector problems on Windows machines. It does this by using a very stripped-down installation of Linux, which features a utility to repair most boot problems and little else. It works with lots of hardware, particularly if that hardware supports UFI, but on the other hand, Boot-Repair-Disk is not open source meaning it is impossible to see exactly what it is doing and how it works with your device's most sensitive pieces.