The MBR (master boot record) file is on the first sector, 512 bytes, of your hard drive. This file contains the hard drive partition table, boot start-up program, and a drive signature. The file resides outside the hard drive partitions on the first sector of the drive and can be repaired using fdisk mbr in DOS or fixmbr command using the Windows recovery console. It would be a good idea to backup the MBR before any changes or modifications are made. There are many MBR utilities that can be found at CNET download for backing up, editing, or making changes to the MBR. The boot start-up or sometimes referred to as the bootstrap program is used to start the operating system, which is a small machine code program that BIOS uses to start-up the operating system. The last part of the MBR files is a 32 bit drive signature that uniquely identifies the hard drive. Since the MBR file is the first file read before the operating system is loaded it has become the target of a number of viruses. Once the MBR file is infected the viruses will copy themselves into memory and control the operating system. Viruses will normally disable in virus protection and infect any files copies to another other device.
To better understand how the Fdisk program operates or the Recovery Console you can find those posts here at this site under the Categories.
FIXMBR
The FIXMBR website has some additional articles you might find useful.