A computer virus is a computer program that can copy itself and infect a computer without permission or knowledge of the user. The term "virus" is also commonly used, albeit erroneously, to refer to many different types of malware and adware programs. The original virus may modify the copies, or the copies may modify themselves, as occurs in a metamorphic virus. A virus can only spread from one computer to another when its host is taken to the uninfected computer, for instance by a user sending it over a network or the Internet, or by carrying it on a removable medium such as a floppy disk, CD, or USB drive. Meanwhile viruses can spread to other computers by infecting files on a network file system or a file system that is accessed by another computer. Viruses are sometimes confused with computer worms and Trojan horses. A worm can spread itself to other computers without needing to be transferred as part of a host, and a Trojan horse is a file that appears harmless. Worms and Trojans may cause harm to either a computer system's hosted data, functional performance, or networking throughput, when executed. In general, a worm does not actually harm either the system's hardware or software, while at least in theory, a Trojan's payload may be capable of almost any type of harm if executed. Some can't be seen when the program is not running, but as soon as the infected code is run, the Trojan horse kicks in. That is why it is so hard for people to find viruses and other malware themselves and why they have to use spyware programs and registry processors.

Most personal computers are now connected to the Internet and to local area networks, facilitating the spread of malicious code. Today's viruses may also take advantage of network services such as the World Wide Web, e-mail, Instant Messaging and file sharing systems to spread, blurring the line between viruses and worms. Furthermore, some sources use an alternative terminology in which a virus is any form of self-replicating malware.

Some malware is programmed to damage the computer by damaging programs, deleting files, or reformatting the hard disk. Other malware programs are not designed to do any damage, but simply replicate themselves and perhaps make their presence known by presenting text, video, or audio messages. Even these less sinister malware programs can create problems for the computer user. They typically take up computer memory used by legitimate programs. As a result, they often cause erratic behavior and can result in system crashes. In addition, much malware is bug-ridden, and these bugs may lead to system crashes and data loss. Many CiD programs are programs that have been downloaded by the user and pop up every so often. This results in slowing down of the computer, but it is also very difficult to find and stop the problem.

A computer virus is a piece of software, usually malicious in nature, which is used to spread itself or other such software from computer to computer. Viruses themselves may cause system damage, loss of valuable data, or can be used to install spyware, rootkits or other malware onto a vulnerable system.
A key way to prevent infection is to have an up-to-date antivirus solution installed on all computers in a network, and to make sure that all of the latest security patches for the computer operating system are installed. Users should also make sure that they can trust the source of software they are downloading from the internet, as many malware types are installed along with other legitimate-looking software.

What is spyware?

Spyware is computer software that is installed surreptitiously on a personal computer to intercept or take partial control over the user's interaction with the computer, without the user's informed consent.

While the term spyware suggests software that secretly monitors the user's behavior, the functions of spyware extend well beyond simple monitoring. Spyware programs can collect various types of personal information, such as Internet surfing habit, sites that have been visited, but can also interfere with user control of the computer in other ways, such as installing additional software, redirecting Web browser activity, accessing websites blindly that will cause more harmful viruses, or diverting advertising revenue to a third party. Spyware can even change computer settings, resulting in slow connection speeds, different home pages, and loss of Internet or other programs. In an attempt to increase the understanding of spyware, a more formal classification of its included software types is captured under the term privacy-invasive software.

In response to the emergence of spyware, a small industry has sprung up dealing in anti-spyware software. Running anti-spyware software has become a widely recognized element of computer security best practices for Microsoft Windows desktop computers. A number of jurisdictions have passed anti-spyware laws, which usually target any software that is surreptitiously installed to control a user's computer.

Spyware is software installed on a computer system that is designed to collect information about the computer user often without their consent or knowledge. This information may result in so-called identity theft, or theft of valuable information (such as bank or credit card details) or proprietary business data.
These days, much of the current spyware is developed by organized crime rings, rather than opportunistic lone individuals and is installed by a virus or another form of malware.

What are rootkits?

A rootkit is malware which consists of a program (or combination of several programs) designed to take fundamental control (in Unix terms "root" access, in Windows terms, "Administrator" access) of a computer system, without authorization by the system's owners and legitimate managers. Access to the hardware (e.g., the reset switch) is rarely required as a rootkit is intended to seize control of the operating system running on the hardware. Typically, rootkits act to obscure their presence on the system through subversion or evasion of standard operating system security mechanisms. Often, they are Trojans as well, thus fooling users into believing they are safe to run on their systems. Techniques used to accomplish this can include concealing running processes from monitoring programs, or hiding files or system data from the operating system.[1]

Rootkits may have originated as regular applications, intended to take control of a failing or unresponsive system, but in recent years have been largely malware to help intruders gain access to systems while avoiding detection. Rootkits exist for a variety of operating systems, such as Microsoft Windows, Linux, Mac OS, and Solaris. Rootkits often modify parts of the operating system or install themselves as drivers or kernel modules, depending on the internal details of an operating system's mechanisms.

Rootkits are programs that install on your system, while keeping themselves, their processes, services and registry keys hidden, to stay invisible from the user. They represent a substantial security risk on home and company networks and are notoriously difficult to find and remove.
Rootkits themselves are normally deployed via another malware infection (such as a Trojan, for instance), and it is therefore highly recommended that computer users have an up-to-date antivirus / anti-spyware system installed and running on their PC.

Trojans Malwares
[url=http://en.wikipedia.org/wiki/Trojan_horse_(computing)]Info[/url]

A program named "waterfalls.scr" serves as a simple example of a trojan horse. The author claims it is a free waterfall screen saver. When run, it instead unloads hidden programs, commands, scripts, or any number of commands without the user's knowledge or consent. Malicious Trojan Horse programs are used to circumvent protection systems in effect creating a vulnerable system to allow unauthorized access to the user's computer.

Hoax Email

E-mail spoofing is a term used to describe fraudulent e-mail activity in which the sender address and other parts of the e-mail header are altered to appear as though the e-mail originated from a different source. E-mail spoofing is a technique commonly used for spam e-mail and phishing to hide the origin of an e-mail message. By changing certain properties of the e-mail, such as the From, Return-Path and Reply-To fields (which can be found in the message header), ill-intentioned users can make the e-mail appear to be from someone other than the actual sender. It is often associated with website spoofing which mimics an actual, well-known website but are run by another party either with fraudulent intentions or as a means of criticism of the organization's activities. The result is that, although the e-mail appears to come from the e-mail indicated in the "From" field (found in the e-mail headers) it actually comes from another e-mail address, probably the same one indicated in the "Reply To" field; if the initial e-mail is replied to, the delivery will be sent to the "Reply To" e-mail, that is, to the spammer's e-mail.

Backdoor
[url=http://en.wikipedia.org/wiki/Backdoor_(computing)]Info[/url]

A backdoor in a computer system (or cryptosystem or algorithm) is a method of bypassing normal authentication, securing remote access to a computer, obtaining access to plaintext, and so on, while attempting to remain undetected. The backdoor may take the form of an installed program (e.g., Back Orifice), or could be a modification to an existing program or hardware device.