Malware is defined as any piece of software that is programmed to do a malicious activity on your computer. Malware can do a variety of different things to your computer such as crash the operating system, download new files, deny you Internet access and a variety of other different activities. Some malware is also programmed to stay dormant until a specific time or date when it will begin a malicious attack.

There are many different types of malware, which include but are not limited to, dialers, trojans, trojan downloaders, computer viruses, adware, and spyware. Each of these different infections has a different purpose, however, it is possible that several different types of malware can combine and do many different things to your computer.

Viruses are what most people have heard of and are widespread but not neccesarily the most virulent of the malware types. A virus is a piece of self replicating code that infects programs or the boot sector of a computer and copies itself anywhere it can in order to infect as many systems as possible.http://support.microsoft.com/kb/129972

A worm is a program that uses various methods on a network to transmit copies of itself to other computers. One method that worms use is to utilize a persons email list and send itself to all the recipients and then when they open the mail the worm infects their system and continues to spread across the internet.http://www.tech-faq.com/computer-worm.html

A trojan horse is a program that masquerades as another program in order to infiltrate a persons computer. When the file is opened it may appear to the user that nothing has happened or the program may appear to do what it was supposed to. What it is actually doing is setting itself up to perform other tasks or to download another program to do things on the computer other than what the user desires.http://www.trojan-horse.info/en/index.aspx

Adware and spyware are programs designed to either display ads or other information without the permission of the user or to obtain information from the users system with out their knowledge or consent. Some just display adds whenever someone is using the computer but others track where you have been or steal your passwords or transmit everything you type to someone for their own use.http://www.adware.info/

Ransomware is designed to take control of the computer system and encrypt all of the files and folders. Once this is done the user is instructed to either send money or purchase certain items in order to receive the password that will release the encrypted filles.http://www.scambusters.org/ransomware.html

Another form of malware masquerades as a virus scanner stating that the computer is infected and that if their software is purchased the problems will be fixed. These programs override legitimate scanners and prevent access to the internet and programs that will fix the problems.http://www.scambusters.org/fakeantivirus.html

These are the major threats that can infect a computer and either cause problems or steal information that can be used by the perpetrators to the detriment of the user. Other threats mostly fall under or near the categories of these particular types of malware.

There are many causes of malware, but some include having too much control on your user account or clicking links that contain a trojan downloader or some other kind of malicious software. There may also be bugs in the code, which is called a vulnerability, and that can be used to exploit the system and download malware to your PC.

Also known as Downadup, Conficker infected millions of PC users in January 2009, exploiting a Microsoft Windows vulnerability. The worm was transfered through a malicious software program which spread by randomly guessing network passwords and transfer through portable storage such as USB devices. Experts said the worm was likely a first step of a multistage attack, turning millions of machines into possible drone or zombie devices.The New York Times: Worm Infects Millions of Computers Worldwide