These are computer viruses that will infect the first or first few sectors of your hard disk or floppy disk, allowing the activation of the virus as the diskette or drive starts up.

What does a boot sector do?

Boot sector viruses infect computer systems by injecting its own code either to the partition table on a hard disk or the boot sector on a floppy disk. When the system boots from the hard disk or diskettes, the virus is executed and installed into the system’s memory. The Boot virus can spread to every disk that the system reads through the memory. These viruses are the most common of all other viruses. The examples of boot viruses are Stoned virus, Michelangelo, Disk Killer, Form, Junkie Virus, and Ohio Virus.

What are the symptoms?

A boot sector virus may cause several problems relating to startup or data retrieval. Data may disappear from entire partitions or the computer suddenly becomes unstable. A failure to startup or to find the hard drive are the common problems that can be noticed when the virus attacks a system.

You may receive an error message on startup saying, "Bad or missing command interpreter. Enter name of command interpreter."

How does it spread?

When the system starts or is attempting to boot, boot sector viruses are spread from an infected floppy disk. Any disk, even if it is non bootable, can transmit the virus into memory if the disk is in the drive while the system is starting up. The virus can also be spread across networks if you are downloading files or trying to open up an infected email attachment.

Precautions you should take:

In order to keep your system and data safe from virus attack, the following tips may be helpful:

• Make sure that you install virus detection software on your computer. Your antivirus program will scan files or your computer's memory for certain patterns that may indicate an infection.

These viruses infect executable program files by adding its own code to the executable file. When you run an infected file, the virus can attach itself to other executable files. The files that are infected by this type of virus are usually with extensions like .COM, .EXE, .SYS (device driver).

Overlay (.OVL) files and Dynamic link library (DLL) files are the file-infecting viruses that are designed for specific program. These are non-executable but are called as executable. The virus becomes active in memory when an infected file is executed. It makes copies of itself and infects files on disk.

Sunday, Cascade, Nemesis, Enigma, and Loki viruses are examples of this type of virus.

Polymorphic viruses change their code so that the antivirus scanners do not detect them. Therefore, this virus produces varied but operational copies of itself. A polymorphic virus encrypts its code in different ways so that it appears differently in each infection.

The examples of polymorphic viruses are Involuntary, Phoenix, Stimulate, Evil, Virus 101, Proud, and Cascade.