Master Boot Record (MBR)
The Master Boot Record (MBR) is a small program that starts when the computer is booting, in order to find the operating system (eg. Windows XP). This complicated process (called the Boot Proces) starts with the POST (Power On Self Test) and ends when the Bios searches for the MBR on the Hard Drive, which is generally located in the first sector, first head, first cylinder (cylinder 0, head 0, sector 1).
The bios will search for the MBR in a few places such as the hard drive, floppy drive, cd/dvd and USB and the order of what is searched can be changed in the CMOS.
Once the bios has found the Master Boot Record then the compter transfers control over to the MBR, which then proceeds to load the operating system. You can create the MBR with Windows/Dos with the command FDISK/MBR.
The Master Boot Record was specifically targeted by viruses early on. A virus in your MBR was fatal to the computer and involved a lot of work for the user to fix the MBR.
Don’t worry though, most MBR viruses infect the computer via a CD or Floppy Disk.