Boot Sector viruses:
The boot sector is the first software loaded onto your computer. This program resides on a disk, and this disk can be either the hard disk inside the computer, a floppy disk or a CD. When a computer is switched on, the hardware automatically locates and runs the boot sector program. This program then loads the rest of the operating system into memory. Without a boot sector, a computer cannot run software.
A boot sector virus infects computers by modifying the contents of the boot sector program. It replaces the legitimate contents with its own infected version. A boot sector virus can only infect a machine if it is used to boot-up your computer, e.g. if you start your computer by using a floppy disk with an infected boot sector, your computer is likely to be infected. A boot sector cannot infect a computer if it is introduced after the machine is running the operating system.
An example of a boot sector virus is Parity Boot. This virus's payload displays the message PARITY CHECK and freezes the operating system, rendering the computer useless. This virus message is taken from an actual error message which is displayed to users when a computer's memory is faulty. As a result, a user whose computer is infected with the Parity Boot virus is led to believe that the machine has a memory fault rather than an disruptive virus infection.