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Virus is a generic term, covering many different types of malicious program: classic viruses, Internet and email worms, Trojans, backdoors, and others.
Whatever its form, a virus is programmed to spread throughout computers and networks by copying itself, almost always unnoticed by the user. The virus payload, or effect, can be annoying, harmful, or even criminal. A virus may be programmed simply to display a humorous message on screen, or to erase all files on the computer, or to steal and distribute confidential data.

The first viruses appeared at the end of the 1960s, or the beginning of the 1970s:
Their effect was relatively limited, for the simple reason that the number of computer users was much smaller than it is today. The increased use of computer technology has led to virus scares becoming an almost everyday occurrence. Some scares are hoaxes. However, real mass attacks have become commonplace, and the consequences are serious, resulting in financial loss for individuals and corporations alike.The number of threats and frequency and speed of attacks is increasing every day. Antivirus protection is therefore a priority for anyone who uses a computer.

Adware is one type of spyware. This is advertising-supported software that displays pop-up advertisements whenever the program is running. The software is usually available via free download from the Internet, and it is the advertisements that create revenue for the company. Although seemingly harmless (aside from intrusiveness and annoyance of pop-up ads), adware can install components onto your computer that track personal information (including your age, gender, location, buying preferences, surfing habits, etc.). Most advertising supported software doesn't inform you that it installs adware on your system, other than via buried reference in a license agreement. In many cases the software will not function without the adware component. Some adware can install itself on your computer even if you decline the offer.

A definition is the set of fingerprints that characterize a piece of spyware. EBIZNETWORKS regularly updates the definitions of known spyware that PCClear uses.

Fingerprints are the unique patterns of files, cookies, and registry entries that spyware installs. These are what PCClear detects and compares to its internal database to determine if spyware is present on your computer.

Spyware is any application that collects information about your computer activities and then sends that information to another individual or company without your knowledge or permission.
Spyware arrives bundled with freeware or shareware, through email or instant messenger, as an Active X install, or by someone with access to your computer. Once on your drive, spyware secretly installs itself and goes to work. Unlike traditional personalization or session cookies, spyware is difficult to detect, and difficult (if not impossible) for the average user to remove.

System monitors
System monitors are applications designed to monitor computer activity to various degrees. These programs can capture virtually everything you do on your computer including recording all keystrokes, emails, chat room dialogue, web sites visited, and programs run. System monitors usually run in the background so that you do not know that you are being monitored. The information gathered by the system monitor is stored on your computer in an encrypted log file for later retrieval. Some programs are capable of e-mailing the log files to another location.
Traditionally, system monitors had to be installed by someone with administrative access to your computer, such as a system administrator or someone that shares your computer. However, there has been a recent wave of system monitoring tools disguised as email attachments or "freeware" software products.

Tracking cookies
Tracking cookies are one type of spyware. These are pieces of information that are generated by a web server and stored on your computer for future access. Cookies were originally implemented to allow you to customize your web experience, and continue to serve useful purpose in enabling a personalized web experience. However, some web sites now issue tracking cookies, which allow multiple web sites to store and access cookies that may contain personal information (including surfing habits, user names and passwords, areas of interest, etc.), and then simultaneously share the information it contains with other web sites. This sharing of information allows marketing firms to create a user profile based on your personal information and sell it to other firms. Tracking cookies are almost always installed and accessed without your knowledge or consent.

Trojan horses
Trojans are one type of spyware. These are malicious programs that appear as harmless or desirable applications. Trojans are designed to cause loss or theft of computer data, and to destroy your system. Some trojans, called RATs (Remote Administration Tools), allow an attacker to gain unrestricted access of your computer whenever you are online. The attacker can perform activities such as file transfers, adding/deleting files or programs, and controlling the mouse and keyboard. Trojans are generally distributed as email attachments or bundled with another software program

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