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VIRUS stands for Vital Information Resource under Siege. Computer viruses are small software programs that are designed to spread from one computer to another for damaging or deleting data on the computer.

The computer system gets infected with a virus through external sources like CD's, Floppy Disk, USB storage devices and Internet. The virus starts spreading when the program will start up or application that is infected with a virus. For example, a word processing program containing a virus, places the virus in the computer memory every time it is run. Once entering the computer memory, the virus may attach itself to other applications, disks or folders. It may also infect a network if it has internet connectivity.

Boot Sector Virus : The Boot Sector Virus affects the boot sector of a floppy or hard disk. It infects the start-up instruction which is required to boot the system. The best way of avoiding boot viruses is to ensure that floppy disks are write-protected and never start the computer with an unknown floppy disk in the disk drive. The examples of boot viruses are Danish Boot, Disk Killer etc.

File Virus : The File viruses attach to executable files such as .com, .exe, and .dll files by inserting virus program instructions into the execution sequence. When the infected file runs, the inserted instructions execute the virus code. After the code finishes executing, the file continues with its normal execution sequence. This happens so quickly that the user will not be aware that the virus has become active. The damage caused by file viruses is very irritating - it displays screen messages, and also destroys data.

Macro Virus : The Macro Viruses do not infect program files. They infect documents. Common targets for many macro viruses are word processors such as Microsoft Word, and spreadsheets such as Microsoft Excel. It inserts unwanted text into documents and reduces the functionality of the computer. Macro viruses that infect Word commonly target the macros that are associated with the Normal .dot template. This template is common to all the word files, so all the word files can be infected.

Trojan horse : Trojan horse is a computer virus which is designed to appear innocent but has been intentionally designed to cause some malicious activities in the system. It can damage files, break password etc. They enter the computer through a network like Internet while downloading or surfing various websites. One of the famous Trojan horse is Black Orifice.

Worm: Worm is one of the common computer threats. It is a program that while running has the ability to spread to other computers on the network using the e-mail addresses found on the computer or by using the Internet, they infect a remote computer. They can use the e-mail address and can send message to everyone without any knowledge.

Resident Virus: Resident Virus is a permanent virus that resides in the RAM memory. It interrupts all the operations executed by the system. It corrupts files and programs that are opened, closed, copied or renamed. The examples of resident virus are Randex, CMJ, Meve and Mrklunky.

Overwrite Virus : Overwrite Virus is designed to delete the information contained in the files. The only way to clean a file infected by an overwrite virus is to delete the file completely, thus losing the original content. The examples of this virus include Way, Trj. Reboot, Trivial.88.D.