What is Computer Virus?
Computer viruses are small software programs that are designed to spread from one computer to another and to interfere with computer operation. A virus might corrupt or delete data on your computer, use your e-mail program to spread itself to other computers, or even erase everything on your hard disk.
Computer viruses are often spread by attachments in e-mail messages or instant messaging messages. That is why it is essential that you never open e-mail attachments unless you know who it's from and you are expecting it.
Viruses can be disguised as attachments of funny images, greeting cards, or audio and video files. Computer viruses also spread through downloads on the Internet. They can be hidden in illicit software or other files or programs you might download.How Computer Viruses Work? 
Here is the general way that viruses work:
- An infected program is run. This is either a program file (in the case of a file-infecting virus) or a boot sector program at boot time. In the case of a Microsoft Word document the virus can be activated as soon as the document that contains it is opened for reading within Microsoft Word. If the "NORMAL.DOT" document template is infected (and this is the most common target of these viruses) then the virus may be activated as soon as Microsoft Word is started up.
- The infected program has been modified so that instead of the proper code running, the virus code runs instead. This is usually done by the virus modifying the first few instructions to "jump" to where the virus code is stored. The virus code begins to execute.
- The virus code becomes active and takes control of the PC. There are two ways that a virus will behave when it is run: direct-action viruses will immediately execute, often seeking other programs to infect and/or exhibiting whatever other possibly malicious behavior their author coded into them. Many file-infector viruses are direct-action. In contrast, memory-resident viruses don't do anything immediately; they load themselves into memory and wait for a triggering event that will cause them to "act". Many file infectors and all boot infectors do this (boot infectors have to become memory resident, because at the time they are executed the system is just starting up and there isn't that much "interesting" for them to do immediately.)
- What exactly the virus does depends on what the virus is written to do. Their primary goals however include replication and spreading, so viruses will generally search for new targets that they can infect. For example, a boot sector virus will attempt to install itself on hard disks or floppy disks that it finds in the system. File infectors may stay in memory and look for programs being run that they can target for infection.
- "Malevolent" viruses that damage files or wreak havoc in other ways will often act on triggers. There are viruses that will only activate on particular days of the year (such as the infamous "Friday the 13th"), or act randomly, say, deleting a file every 8th time they are run. Some viruses do nothing other than trying to maximize their own infection to as many files and systems as possible.Most Common Types of Viruses and Other Malicious Programs
- Resident Viruses
This type of virus is a permanent which dwells in the RAM memory. From there it can overcome and interrupt all of the operations executed by the system: corrupting files and programs that are opened, closed, copied, renamed etc.
Examples include: Randex, CMJ, Meve, and MrKlunky. - Multipartite Viruses
Multipartite viruses are distributed through infected media and usually hide in the memory. Gradually, the virus moves to the boot sector of the hard drive and infects executable files on the hard drive and later across the computer system. - Direct Action Viruses
The main purpose of this virus is to replicate and take action when it is executed. When a specific condition is met, the virus will go into action and infect files in the directory or folder that it is in and in directories that are specified in the AUTOEXEC.BAT file PATH. This batch file is always located in the root directory of the hard disk and carries out certain operations when the computer is booted. - Overwrite Viruses
Virus of this kind is characterized by the fact that it deletes the information contained in the files that it infects, rendering them partially or totally useless once they have been infected.
The only way to clean a file infected by an overwrite virus is to delete the file completely, thus losing the original content.
Examples of this virus include: Way, Trj.Reboot, Trivial.88.D. - Boot Virus
This type of virus affects the boot sector of a floppy or hard disk. This is a crucial part of a disk, in which information on the disk itself is stored together with a program that makes it possible to boot (start) the computer from the disk.
The best way of avoiding boot viruses is to ensure that floppy disks are write-protected and never start your computer with an unknown floppy disk in the disk drive.
Examples of boot viruses include: Polyboot.B, AntiEXE. - Macro Virus
Macro viruses infect files that are created using certain applications or programs that contain macros. These mini-programs make it possible to automate series of operations so that they are performed as a single action, thereby saving the user from having to carry them out one by one.
Examples of macro viruses: Relax, Melissa.A, Bablas, O97M/Y2K. - Directory Virus
Directory viruses change the paths that indicate the location of a file. By executing a program (file with the extension .EXE or .COM) which has been infected by a virus, you are unknowingly running the virus program, while the original file and program have been previously moved by the virus.
Once infected it becomes impossible to locate the original files. - Polymorphic Virus
Polymorphic viruses encrypt or encode themselves in a different way (using different algorithms and encryption keys) every time they infect a system.
This makes it impossible for anti-viruses to find them using string or signature searches (because they are different in each encryption) and also enables them to create a large number of copies of themselves.
Examples include: Elkern, Marburg, Satan Bug, and Tuareg. - File Infectors
This type of virus infects programs or executable files (files with an .EXE or .COM extension). When one of these programs is run, directly or indirectly, the virus is activated, producing the damaging effects it is programmed to carry out. The majority of existing viruses belongs to this category, and can be classified depending on the actions that they carry out. - Encrypted Viruses
This type of viruses consists of encrypted malicious code, decrypted module. The viruses use encrypted code technique which make antivirus software hardly to detect them. The antivirus program usually can detect this type of viruses when they try spread by decrypted themselves. - Companion Viruses
Companion viruses can be considered file infector viruses like resident or direct action types. They are known as companion viruses because once they get into the system they "accompany" the other files that already exist. In other words, in order to carry out their infection routines, companion viruses can wait in memory until a program is run (resident viruses) or act immediately by making copies of themselves (direct action viruses).
Some examples include: Stator, Asimov.1539, and Terrax.1069 - Network Virus
Network viruses rapidly spread through a Local Network Area (LAN), and sometimes throughout the internet. Generally, network viruses multiply through shared resources, i.e., shared drives and folders. When the virus infects a computer, it searches through the network to attack its new potential prey. When the virus finishes infecting that computer, it moves on to the next and the cycle repeats itself.
The most dangerous network viruses are Nimda and SQLSlammer. - Nonresident Viruses
This type of viruses is similar to Resident Viruses by using replication of module. Besides that, Nonresident Viruses role as finder module which can infect to files when it found one (it will select one or more files to infect each time the module is executed). - Stealth Viruses
Stealth Viruses is some sort of viruses which try to trick anti-virus software by intercepting its requests to the operating system. It has ability to hide itself from some antivirus software programs. Therefore, some antivirus program cannot detect them. - Sparse Infectors
In order to spread widely, a virus must attempt to avoid detection. To minimize the probability of its being discovered a virus could use any number of different techniques. It might, for example, only infect every 20th time a file is executed; it might only infect files whose lengths are within narrowly defined ranges or whose names begin with letters in a certain range of the alphabet. There are many other possibilities. - Spacefiller (Cavity) Viruses
Many viruses take the easy way out when infecting files; they simply attach themselves to the end of the file and then change the start of the program so that it first points to the virus and then to the actual program code. Many viruses that do this also implement some stealth techniques so you don't see the increase in file length when the virus is active in memory.
A spacefiller (cavity) virus, on the other hand, attempts to be clever. Some program files, for a variety of reasons, have empty space inside of them. This empty space can be used to house virus code. A spacefiller virus attempts to install itself in this empty space while not damaging the actual program itself. An advantage of this is that the virus then does not increase the length of the program and can avoid the need for some stealth techniques. The Lehigh virus was an early example of a spacefiller virus. - FAT Virus
The file allocation table or FAT is the part of a disk used to connect information and is a vital part of the normal functioning of the computer.
This type of virus attack can be especially dangerous, by preventing access to certain sections of the disk where important files are stored. Damage caused can result in information losses from individual files or even entire directories. - Worms
A worm is technically not a virus, but a program very similar to a virus; it has the ability to self-replicate, and can lead to negative effects on your system and most importantly they are detected and eliminated by antiviruses.
Examples of worms include: PSWBugbear.B, Lovgate.F, Trile.C, Sobig.D, Mapson. - Trojans or Trojan Horses
Another unsavory breed of malicious code (not a virus as well) are Trojans or Trojan horses, which unlike viruses do not reproduce by infecting other files, nor do they self-replicate like worms. - Logic Bombs
They are not considered viruses because they do not replicate. They are not even programs in their own right but rather camouflaged segments of other programs.
Their objective is to destroy data on the computer once certain conditions have been met. Logic bombs go undetected until launched, and the results can be destructive.