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    There is a broad range of ways that malicious software (malware) can infect your computer. And once your computer has become infected, the results can range from minor annoyance, to data loss, to a completely unusable machine. Most malware, as a minimum, will try to use your computer to infect other computers, as well as open doorways to allow other infectious code onto your machine. Once a few bad seeds have been planted in your computer, the speed with which other problems can crop is staggering. This is why prevention and basic education about the threats is one of the best ways to combat the problem.

    If you read the headlines in the daily paper, or watch the nightly news, you have probably, at least, heard mention of the following terms: virus, trojan, worm, backdoor, spyware, adware, and virus hoax (to name a few). But what does it all mean?

Malware – Malicious software. The broadest heading for any bad software. (Mal is the Latin root for “bad”)

 Virus – A computer virus is piece of code that infects files or programs. When an infected file is exposed to an uninfected file or an uninfected program, it can spread the infection – just like a virus. In addition to this process of spreading and infecting, a virus will usually carry some “payload”. The payload is usually some additional action the virus takes once it has infected a computer. The payload can do almost anything imaginable, ranging from deleting all your files to simply setting your clock behind one hour.

 Trojan Horse – The Trojan Horse is so named because of it’s similarity to the Trojan Horse of ancient history – believed to be a gift from the Gods and let into to the city of Troy, only to have an army of Greeks to emerge from it’s belly. A Trojan is a program that you put on your computer and run, believing it to be something it is not. You may download and run what you think is a weather or music program, but in fact it is stealing your passwords and sending them to bad people. It may even appear to be a perfectly legitimate program and work the way you thought it would. A Trojan differs from a virus in that it does not replicate itself, but relies on the user to install it.

 Worm – Worms are what have been receiving the greatest amount of media attention recently, due to the absolute speed with which they move and the total prevalence of them on the Internet. An unpatched Windows XP  machine plugged into the internet will be infected with one or several worms within minutes or seconds. A worm acts much like a virus in that it A) self-replicates, and B) delivers some payload. However, while a virus is typically imbedded in a file, and can only be transmitted by transmitting that file (email for example), a worm is less tied down and can jump in and out of files and from computer to computer at will. A worm doesn’t need the user to do anything at all; all it needs is an internet connection and unprotect computers.

 Virus Hoax – A virus hoax is little more than a glorified chain letter. You may receive an email claiming “There is some awful virus you need to warn all your friends about, so send this letter on to them.” A virus hoax can be just as pervasive as a virus; it replicates itself by tricking people into sending copies to all their friends. In a worst case, a virus hoax can deliver a payload as well. It may instruct the user to delete some viral files, like system32.dll. This is, of course, an important operating system file, but the uninformed user may be fooled into deleting it, thereby wreaking havoc on their own system. Virus hoaxes rely on a user’s willingness to blindly follow instructions. While this may sound silly, history has shown us that people are all too willing to do so. The simplest solution to this particular type of malware is common sense. Don’t forward these emails. Don’t follow unsolicited instructions.

 Backdoor – A backdoor is a payload that can be delivered by a trojan or a worm. It is a secret entrance (as the name suggests) into your computer that can be exploited by bad person.

 Spyware – Spyware is essentially a mild form a trojan. Typically you will install some program, and without your knowledge it will monitor your internet activity, what kinds of things you purchase online, etc and report back to the bad guys to give them what they consider to be marketing data. If the spyware did worse things, like steal your passwords, you would call it a trojan.

The terms 'adware' and 'spyware' are often used interchangeably, and the distinction between the two is often quite unclear.

 Adware – Adware is similar to spyware in that, in theory, it monitors your internet activity and then presents you with (usually popup) advertisements for things that it thinks your inclined to like. In practice, these programs are not nearly so graceful- in large numbers, the advertisements they spawn and the system resources they require to run can render your computer completely unusable.