A virus is usually designed to replicate and distribute itself.  In the process it may cause damage to your computer, but not always.  Most frequently, a virus is distributed via e-mail (usually as an attachment).  When you receive this e-mail and execute the attachment (either intentionally or not), the virus code will operate and perform it's function...usually without your knowledge.

Some computer viruses are transported via programs that are available on the internet as 'downloads'.  These downloaded programs are copied from the internet site and then reside on your hard drive.  Once the downloaded program is executed, the virus code operates and performs it's function.

Here's an article that was sent to us, discussing why a virus or worm is created:

Virus Awareness Month: Who Writes This Stuff?

After hearing about a virus, you might be wondering who writes viruses and why do they do it? This is not a simple question, and has actually been the subject of many studies all around the globe. (Click the link below to find an article written on that very subject.) People who write viruses appear to come from all walks of life. The most common excuse hackers give for writing destructive viruses is the following: ".to expose the frailty of the system." Perhaps, but this bothersome as well as pathetic excuse to engage in sociopathic behavior exonerates the writer only in his/her own mind and distances him/her from the damage s/he causes.

Simply put: There is absolutely no legitimate reason to write intentionally destructive programs other than to inflict damage to other people. Viruses, worms, Trojan horses, and other destructive programs are not a form of expression, a show of talent, or a hobby; writing any or all is a degenerative act propagated by misguided need for respect that only seems legitimate to the author.

I dare any of these virus writers to face the thousands of persons who have had emails, pictures, notes, books, ideas, and dreams destroyed and explain that they were simply trying to "expose the frailty of the system."

You can go to a number of internet sites in order to determine if you are dealing with a hoax, here a a few:

W95.Klez Virus/Worm
This little guy is a pretty nasty one. It's been raising havoc not only on the internet, but on personal comptuers due to it's nasty features. There's actually a number of 'variants' of this virus/worm, they are all very simlar.

One of the worst features is that it replicates itself by sending off e-mail when you are on-line, usually without you being aware of this. This is very similar to other virus activity, however in this case it will often use one of the e-mail addresses in your address book as the 'From:' location...in effect hiding the origin of the e-mail, making it look like it came from that person instead of you! This causes the 'other' person to look as if they are the one spreading the virus.

Another nasty feature of this virus/worm is that it tries to prevent you from loading, installing, or updating your virus protection software. I have personally seen a computer where the installation of Norton Anti-Virus was halted because the computer was infected by this virus/worm and it deleted executable files during the setup! Very clever! You can get help from the virus software sites (a tool to remove the virus so you can install your virus protection software).

Here's an article that goes into more detail on this virus/worm: