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.
you can type FIXMBR at the prompt and the MBR (master boot record) is rebuilt and replaced with a new MBR file. This can usually correct some of the startup problems with a corrupted or infected MBR record. Since the MBR record is the first 512 bytes read on the boot hard drive it is also a target for many viruses. If the MBR gets infected the virus can move into memory and control all other boot processes and disable any anti-virus programs. Most anti-virus programs like AVG will detect and stop viruses, but if you boot the computer to an infect device, such as, a floppy or CD, it can infect the MBR. Just rebooting the computer to Recovery Console and using FIXMBR will remove the virus and correct the problem. One WARNING that should be mentioned is that dual boot systems where you have more than one bootable operating system on a hard drive can be affected so that you lose the ability to boot the alternate operating system. You might want to backup the MBR record first just so you can restore it if needed. I also have a FIXMBR YouTube video you might find helpful.