Threats that can effect your Computer
There are many different threats to your computer, as described below. Some forms of these threats are "mild" in nature, causing minor disruptions in the operation of your computer. On the other hand, some forms of these threads can be "serious" in nature, which can stop your computer from working altogether. This document is designed to inform you about the most common threats, so that you will be in a better position to deal with them.
Malware is a general term that describes just about any type of threat that can prevent or alter the way you use your computer. All of the terms below are specific forms of Malware.
A computer virus is a small software program that spreads from one computer to another and interferes with computer operation. A computer virus might corrupt or delete data on a computer, use an email program to spread the virus to other computers, or even delete everything on the hard disk. Computer viruses are frequently spread by attachments in email messages or by instant messaging messages. Therefore, you must never open an email attachment unless you know who sent the message or you are expecting the email attachment. Viruses can be disguised as attachments of funny images, greeting cards, or audio and video files. Computer viruses also spread through downloads on the Internet. They can be hidden in pirated software or in other files or programs that you might download.
A worm is computer code that spreads without user interaction. Most worms begin as email attachments that infect a computer when they're opened. The worm scans the infected computer for files, such as address books or temporary webpages, that contain email addresses. The worm uses the addresses to send infected email messages, and frequently mimics (or spoofs) the "From" addresses in later email messages so that those infected messages seem to be from someone you know. Worms then spread automatically through email messages, networks, or operating system vulnerabilities, frequently overwhelming those systems before the cause is known. Worms aren't always destructive to computers, but they usually cause computer and network performance and stability problems.
A trojan horse is a malicious software program that hides inside other programs. It enters a computer hidden inside a legitimate program, such as a screen saver. Then it puts code into the operating system that enables a hacker to access the infected computer. Trojan horses do not usually spread by themselves. They are spread by viruses, worms, or downloaded software
Spyware can install on your computer without your knowledge. These programs can change your computer’s configuration or collect advertising data and personal information. Spyware can track Internet search habits and can also redirect your web browser to a different website than you intend to go to.
Rogue Security Software
A rogue security software program tries to make you think that your computer is infected by a virus and usually prompts you to download or buy a product that removes the virus. The names of these products frequently contain words like Antivirus, Shield, Security, Protection, or Fixer. This makes them sound legitimate. They frequently run right after you download them, or the next time that your computer starts. Rogue security software can prevent applications, such as Internet Explorer, from opening. Rogue security software might also display legitimate and important Windows files as infections.
(The source of this information is Microsoft - https://support.microsoft.com/en-us/kb/129972)