Virus
Virus is a small program which destroys the computer files and documents.
A computer virus is a small software prohson that spreads from one computer to anothe computer and that interferes with computer operation. A computer virus may corrupt or delete data on a computer. Use an e-mail program to spread the virus to other computers. Or even delete everything on the hard disk.
Computer viruses are most easily spread by attachments in e-mail messages or by instant messaging messages. Computer viruses can be disguised as attachments of funny images, greeting cards, or audio and video files. Computer viruses also spread by using downloads on the internet. Computer viruses can be hidden in pirated software or in other files or programs that you may download.

Symptoms of a computer virus
¤The computer runs slower than usual.
¤The computer stops responding, or it locks up frequently.
¤The computer restartr on its own. Additionally, the computer does not run as usual.
¤Applications on the computer do not work correctly.
¤Disks or disk drives are inaccessible.
¤You see unusual error messages.
¤You see distorted menus and dialog boxes.
¤There is a double extension on an attachment that you recently opened, such as .jpg, .Vbs, .Gif, .Exe.
¤Strange sounds or music plays from the speakers unexpectedly.
¤A program disappears from the computer even though you did not intentionally remove the program.

Types of Viruses
Depending on the place where the viruses is resident they can be divided into the following types.
1.File viruses
2.Boot viruses
3.Macro viruses
4.Network viruses

File viruses
either infect executables in various ways, or create file doubles, or use file system specific features.

Boot viruses
either save themselves in disk boot sector, or to the master Boot Record, or change the pointer to an active boot sector.

Macro viruses
infect document files, electronics spreadsheets and databases of several popular software packages.

Network viruses
use protocols and commands of computer network or e-mail to spread themselves.

Based on their destructive capabilities viruses can be further divided as follows:
Harmless
that is having on effect on computing(except for some lowering of free disk space as a result of propagation).

Dangerous viruses
which may seriously disrupt the computer's work;

Very dangerous
the operating algorithms of which intentionally contain routines which may lead to losing data, data destruction, erasure of vital information in system areas, and even according to one of the unconfirmed computer legends inflict damage to the moving mechanical parts by causing resonance in some kinds of HDDs.

Bomb
A bomb is a program that resides silently in a computer's memory untill it is triggered by a specific condition,such as a date.

worm
A worm is a destructive program that propagates itself over a network, reproducing as it goes.

Trojan horse
A trojan horse is a malicious program that passes itself off as a benign applicatio, it cannot reproduced itself and, like a virus, must be distributed by diskette or electronic mail.

How Does a Computer Get a Virus?
a virus can be attached to any file that you copy to your computer. If you download files from the internet or copy programs or files from friends on floppy disks or pirated software, you are very susceptible to viruses. Many viruses are spread through e-mail.

Prevention of Viruses Before They Attack
a computer virus can do extensive damage. Many viruses do weird little things that adversely affect your computer. The most usual symptom of a computer virus is erratic behavior. If your computer starts to act a little weird, the first thing to do is to check for a virus.

If you are smart. You will stop viruses before they enter your computer. You can do this by purchasing a good virus protection program. Such a program will check all files for viruses. Once installed, an antivirus program can be set to work in the background. It will check all files before they enter your computer and will alert you if a virus is detected before it contaminates your system. If a virus is detected, your antivirus program will guarantine as eliminate is so that it cannot harm your computer.