What is a trojan, virus and worm

Often antivirus alerts in the press and media refer to trojans, viruses and worms, but is there any difference or are they just different names for the same thing.

Trojan

Usually a trojan refers to software that is not what it seems. It may claim to show pictures or more recently many trojans claim to remove spyware from your computer. However they all have some hidden payload, which may load spyware or other viruses onto your computer. Sometimes they seem to do what they claim, such as display and image, or pretend to have found spyware, but this is just to hide their real purpose.

Virus

Viruses are the most common type of threat, and are similar to trojans. The key difference is that viruses usually infect other programs on your computer. So whenever you run a program infected with the virus, you are also running the virus again. The key difference between a virus and a trojan however is that a virus is designed to replicate, either by infecting other programs and documents on your computer, or by sending copies of itself out to everyone in your email address book.

Viruses that use email to distribute themselves tend to spead much faster than those that require infected documents to spread.

Worm

Like a virus, a worm is designed to replicate itself and infect other computers, but unlike a worm, is able to do this without any human interaction. For a virus to infect your computer, you usually need to run the infected attachment in your email. A worm needs no such help, and will usually connect from computer to computer infecting every PC it can, which inturn go on to infect more computers. Worms can be the fastest spreading of all threats, especially if the vulnerability the worm is exploiting (in order to spread) is widespread.

The most susceptible versions of windows to worms are NT, 2000, XP and 2003, however a good firewall can protect your computer from worms depending on the exploit being used to spread.

Windows Guide