Computer virii are programs that are written by people for various reasons. Some viruses have started life as a "proof of concept" virus to expose a security hole or prove a point. Many of the virii authors try to claim that even if it is not the truth. The end result when one of these "proof of concept" virii gets loose is the same a deliberate code that tried to get into your system.
There are more types of computer virii out in the world than I can go into. Some cause a minor annoyance to the users of the system. The ones you should worry about in your sleep cause passwords, personal information or announce access of a system to be sent back to specific email accounts. A current example of such a virus is Bugbear Others run so poorly that your pc crashes easily or even fail to run at all.
The types of virii that announce your pc is now accessable are refered to as back door virii. When your pc has a back door virus, it has been setup so it receives commands from a central server(s). The infected pc can then be used for denial of service attacks (refered to as DDOS or DOS attacks) or spreading of virii to other machines. Consider this point, if you let your machine get infected by a back door virus, should you be held responsable (even to just limited extent) for damage caused to other systems? If you have a computer on the internet, you could become a vector for distributing a virus. An extreme parallel would be this, if you have a serious case of the flu, whould you go to work and run the risk os spreading it around? If you did, how would your co workers feel? I bet not good. In the grand scheme of the internet, we are all co-workers. Each pc on the net from the lowly work station to the mega horsepower server are players on the internet. Each can positivly or negativly affect the overall network.
Examples of how extreme backdoor virii can be are the attacks on Yahoo, the White House site and colocation facilities. The server that the www.kc5goi.net domain is on has been a victum of such an attack. It can knock even a robust server down to just causing timeouts or long return times on web sites and email.
Another method a machine can be compromised and taken over is via hacking directly. I will not go into that here, but if you have a internet connection that is on all the time (cable, two way satillite or DSL) you really should look at a firewall or keep your router firmware up to date.
Virii can come to you in the form of a notification and not actual computer code. This type of virus is called a good samaritan virus. They normally get started by some rouge person sending out an email suggesting that you find a file, normally in your c:\windows\system or c:\windows\system32 directory, deleting it and rebooting. The effect normally reappears after reboot (if you can reboot) or when you lauch a program like Internet Explorer. Another effect this type of virus has is causing congestion on the net, for example Badtimesvirus.
By now you have asked the question of how do I protect myself or do I have enough protection. First let me say this and it only encompasses virii, NOT hacked systems. Nothing is bullet proof, even anti virus software. The recent KLEZ virus has the abilty to disable most types and vendors of anti-virus software. Once it was disabled, the system was had for the most part. There are patches for most AV software to fix the problem. Next, install anti-virus software. I do have a favorite, but I am not in the business of making that kind of recommendation. Keep what ever AV software you purchased up to date, if you do not keep it up to date, then you wasted your time and money. Most vendors sell subscriptions for updates. Get the subscription and keep it up to date. I recommend you either tell the software to update automatically or make a habit of doing it atleast once a week, no less. Make sure that you have the AV software running and real time protection ENABLED. If you disable it, why did you bother to get it and install it to begin with. I have been to customer sites that have software installed but it will not run correclty if AV software is ENABLED. To the authors of such software, this is unforgivable. All it takes is to forget one time to ENABLE the AV software, check for messages and get infected.
Some ISP's offer virii scanning as a part of their services, this does not take the issue of protection off of your shoulders. Before the Internet, virii was spread by floppy disks and cds. Today you can get a virus but floppys, cds, websites, email, Office documents and even PDF documents.
Short story, get yourself AV software, install it, update it regularly, ENABLE it at all times, and scan regularly. Run Windows updates on Windows pcs, run Office updates for Microsoft Office 2000 and up, update your MAC, keep your Linux patched and AV software on. Linux and Macs are just as able to get a virus as any Microsoft product. Microsoft happens to be the favorite right now.
But Guy, I run a Mac or Linux and am untouchable by virii. This is only semi correct. While the payload may not run on your system, you can still pass it on (the virus) like a human based virus. Just because I am not sick does not mean I can not spread it.
Scheduling scans in the middle of the night or during a time of day you know that the system is not in use is suggested.
Guy, I got angry emails and phone calls from my friends saying I sent them a virus. I checked my system and I show to be virus free. What is the deal?
There is a very good chance these days you did not send the email(s) you are being accused of. I refer you to this article at Symantecs site. Do not forget the Klez is spread by email. It will look at your address box, copy an address in it and claim that the copied address if who sent it. In the properies of the email is the truth, but you have to know what you are looking for. Since then, many other virii have dupicated this method.
Note, these are not endorsments.
Mc Afee Watch for popup adds for their other software. I dislike popup adds.