A boot sector virus infects the master boot record (MBR) of a hard drive or the boot sector of a hard drive or removable media, such as floppy diskettes. The boot sector is an area at the beginning of a drive or disk where information about the drive or disk structure is stored. Boot sectors contain boot programs that are run at host startup to boot the operating system.
The MBR of a hard drive is a unique location on the disk where a computer's basic input/output system (BIOS) can locate and load the boot program. Removable media, such as floppy disks, need not be bootable to infect the system; if an infected disk is in the drive when the computer boots, the virus could be executed. Boot sector viruses are easily concealed, have a high rate of success, and can harm a computer to the point of making it completely inoperable. Symptoms of boot sector virus infection on a computer include an error message during booting or the inability to boot. Form, Michelangelo, and Stoned are examples of boot sector viruses.