Viruses and Trojan Horses are the most significant nuisance of the Internet.

A computer virus is a program that replicates. To do so, it will need to attach itself to other program files (for example, .exe, .com, .dll, .mdb) and execute whenever the host program executes. Beyond simple replication, a virus almost always seeks to fulfill another purpose: to cause damage.
Called the damage routine, or payload, the destructive portion of a virus can range from overwriting critical information kept on your hard disk's partition table to scrambling the numbers in your spreadsheets to just taunting you with sounds, pictures, or obnoxious effects.

It’s worth bearing in mind, however, that even without a damage routine, left unabated viruses will continue to propagate--consuming system memory, disk space, slowing network traffic and generally degrading performance. Besides, virus code is often buggy and can also be the source of mysterious system problems that take weeks to understand. So, whether it was written to be harmful or not, a virus on your system can lead to instability and should not tolerated.

Some viruses, in conjunction with logic bombs, do not make their presence known for months. Instead of causing damage right away, these viruses do nothing but replicate until the preordained trigger day or event when they unleash their damage routines across the network.

But not only Viruses can damage your system. Much more dangerous are each kind of programm files, you download from internet or Newsnet because no Software will warn you for any damage command. Do you really know what this progams are able to do?

I will tell you a story, that happend to me some month ago:
I downloaded a small Application called "Nettoop" from Newsgroup. The Program should be able to play broken Movie-files.
The Application comes with a self-extracting Installation file (.exe). When I started the Installation, it asked me wether I accept the Licence or if I want to read the Readme.wri. When I canceled the Installation for searching the Readme, I noticed that my network-connection was going to connect the Internet and is waiting for confirmation. I canceled the connection.
Even when i started any .exe file, my system wants to establish an internet-connection. I recognized, that the Installation has overrided the .exe entry of my registry. Anytime when an .exe file was started, a small program called run.exe was started too. This run.exe starts the Internet connection. I don´t know what for, and in reality I don´t want to know.

Actually i have had good luck. If this "run.exe" had been a "format.exe"... 2 seconds later, my HD would have gone for ever.

This example should show you to be very coutious in loading any executable file from newsgroup.

Better never load any executable file from unknown adresses.