Malicious software comes in many forms. All forms have certain things in common, though. For one, they’re invisible — you don’t even know they’re there. For another, they all do something bad, something you don’t really want happening on your computer. Third, they’re all written by human programmers to intentionally do these bad things. The differences have to do with how they spread and what they do after they’re on your computer. I tell you about the differences in the sections to follow.
Viruses and worms
Viruses and worms are self-replicating programs that spread from one computer to the next, usually via the Internet. A virus needs a host file to spread from one computer to the next. The host file can be anything, though viruses are typically hidden in e-mail attachments and programs you download.
A worm is similar to a virus in that it can replicate itself and spread. However, unlike a virus, a worm doesn’t need a host file to travel around. It can go from one computer to the next right though your Internet connection. That’s one reason it’s important to always have a firewall up when you’re online — to keep out worms that travel through Internet connections.
The harm caused by viruses and worms ranges from minor pranks to serious damage. A minor prank might be something like a small message that appears somewhere on your screen where you don’t want it. A more serious virus might erase important files, or even try to erase all your files, rendering your computer useless.
Spyware and adware
Spyware and adware is malware that’s not designed to specifically harm your computer. Rather, it’s designed to help people sell you stuff. A common spyware tactic is to send information about the Web sites you visit to computers that send out advertisements on the Internet. That computer analyzes the Web sites you visit to figure out what types of products you’re most likely to buy. That computer then sends ads about such products to your computer.
Adware is the mechanism that allows ads to appear on your computer screen. When you get advertisements on your screen, seemingly out of the clear blue sky, there’s usually some form of adware behind it. Spyware and adware often work in conjunction with one another. The adware provides the means to display ads. The spyware helps the ad server (the computer sending the ads) choose ads for products you’re most likely to buy.
Trojan horses and rootkits
You may have heard the term Trojan horse in relation to early mythology. The story goes like this. After 10 years of war with the city of Troy, the Greeks decided to call it quits. As a peace offering, they gave to the people of Troy a huge horse statue named the Trojan horse. While the people of Troy were busy celebrating the end of the war, Greek soldiers hidden inside the horse snuck out and opened the gates to the city from inside. This allowed other Greek soldiers, lying in wait hidden outside the city, to storm into the town and conquer it. (This is definitely a case in which it would have been wise to look a gift horse in the mouth.)
A Trojan horse is a program that works in a similar manner. In contrast to other forms of malware, a Trojan horse is a program you can actually see on your screen and use. On the surface, it does do something useful. However, hidden inside the program is some smaller program that does bad things, usually without your knowledge.
A Trojan horse can also be a program that hides nothing but could be used in bad ways. Take, for example, a program that can recover lost passwords. On the one hand, it can be a good thing if you use it to recover forgotten passwords from files you created yourself. But it can be a bad thing when used to break into other people’s password-protected files. A rootkit is a program that is capable of hiding itself, and the malicious intent of other programs, from the user and even from the system. As with Trojan horses, not all rootkits are inherently malicious. However, they can certainly be used in malicious ways. Windows 7 protects your system from rootkits on many fronts, including Windows Defender.
Source of Information : Windows 7 Bible (2009)
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