Malicious software, commonly known as malware, is any software that brings harm to a computer system. It can be used to disrupt computer operation, gather sensitive information, or gain access to private computer systems. Malware can be in the form of worms, viruses, Trojans etc., which steal protected data, delete documents or add software not approved by a user. Malware takes partial to full control of our computer to do whatever the malware creator wants. Most malware requires the user to initiate its operation. Some form of attacks includes attachments in e-mails, browsing a malicious website that installs software after the user clicks ok on a pop-up.


This type of Malware uses network resources for spreading. This class was called worms because of its peculiar feature to creep from computer to computer using network, mail and other informational channels. Worms intrude our computer, calculate network addresses of other computers and send to these addresses its copies. Many worms that have been created are designed only to spread, and do not attempt to change the systems they pass through. The biggest danger with a worm is its capability to replicate itself on your system, so it could send out hundreds or thousands of copies of itself, creating a huge devastating effect.

One example would be for a worm to send a copy of itself to everyone listed in your e-mail address book. Then, the worm replicates and sends itself out to everyone listed in each of the receiver's address book, and the manifest continues on down the line. Due to the copying nature of a worm and its capability  to travel across networks the end result in most cases is that the worm consumes too much system memory (or network bandwidth), causing web servers, network servers and individual computers to stop responding.

Father Christmas is an example of worm. It was distributed in 1987 and was

designed for IBM networks. It was an electronic letter instructing recipient to save it and run it as a program that drew Christmas tree, printed ―Merry Christmas!‖ It also checked address book, list of previously received email and sent copies to each address. The worm quickly overwhelmed the IBM networks and forced the networks and systems to be shut down



A computer virus is a program that inserts itself into one or more files and then performs some (possibly null) action. Computer virus works in two phases. The first phase, in which the virus inserts itself into a file, is called the insertion phase. The second phase, in which it performs some action, is called the execution phase. Almost all viruses are attached to an executable, which means the virus may exist on our computer but it actually cannot infect your computer unless we run or open the malicious program. It is important to note that a virus cannot be spread without a human action, (such as running an infected program) to keep it going. Because a virus is spread by human action people will unknowingly continue the spread of a computer virus by sharing infecting files or sending emails with viruses as attachments in the email.

The Brain (or Pakistani) virus, written for IBM PCs is an example of this category. It is thought to have been created in early 1986 but was first reported in the United States in October 1987. It alters the boot sectors of floppy disks, possibly corrupting files in the process. It also spreads to any uninfected floppy disks inserted into the system.

Torjan Horse

Trojan horses are the files that claim to be something desirable but, in fact, are malicious code or logic. The Trojan Horse, at first glance will  appear  to  be useful software but will actually do damage once installed or run on your computer. Receivers of a Trojan Horse are usually tricked into opening them because they appear to be receiving legitimate software or files from a legitimate source. When a Trojan is activated on your computer, the results can vary. Some Trojans are designed to be more annoying  than  malicious  like  changing our desktop, adding silly active desktop icons etc. Sometimes they can cause serious damage by deleting files and destroying information on your system.

Trojans are also known to create a backdoor on your computer that gives malicious users access to your system, possibly allowing confidential or personal information to be compromised. Unlike viruses and worms, Trojans do not reproduce by infecting other files nor do they self-replicate. A program named "waterfalls.scr" serves as a simple example of a Trojan Horse. The author claims it is a free waterfall screensaver. When run, it instead unloads hidden programs, commands, scripts, or any number of commands without the user's knowledge or consent.


Spyware is any software installed on your PC that collects your information without your knowledge, and sends that information back to the creator so they can use your personal information in some nefarious way. This could include keylogging to learn your passwords, watching your searching habits, changing out your browser home and search pages, adding obnoxious browser toolbars, or just stealing your passwords and credit card numbers.

Since spyware is primarily meant to make money at your expense, it doesn't usually kill your PC—in fact, many people have spyware running without even realizing it, but generally those that have one spyware application installed also have a dozen more. Once you've got that many pieces of software spying on you, your PC is going to become slow.

Did you know? What is backdoor?

A backdoor in   a computer system   is   a    method    of    bypassing normal authentication, securing unauthorized remote access to a computer, obtaining access to plain text, and so on, while attempting to remain undetected.