Definition: A virus is a software program, script, or macro that has been designed to infect, destroy, modify, or cause other problems with a computer or software program. Users can protect themselves by installing an anti-virus protection program.
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Also see: anti-virus, Boot sector virus, CMOS virus, Companion virus, Executable virus, Hoax, Intruder, Logic bomb, Macro virus, MBR virus, Multipart virus, Non resident virus, Overwrite virus, Polymorphic virus, Resident virus, Stealth virus, Trojan horse, Vaccine, Security definitions, Worm, Zoo
Definition: A Trojan horse is a program that unlike a virus, contains or installs a malicious program (sometimes called the payload or 'trojan') while under the guise of being something else. The term is derived from the classical myth of the Trojan Horse. Trojan horses may appear to be useful or interesting programs (or at the very least harmless) to an unsuspecting user, but are actually harmful when executed. A simple example of a trojan horse would be a program named "waterfalls.scr" claiming to be a free waterfall screensaver which, when run, instead would allow access to the user's computer remotely.
Method: Once within the computer or network, it causes that computer or network harm by replicating itself and deleting, modifying, distributing, or otherwise manipulating the data.