There are several elements to a good virus defense. The most important element requires some self-control ”you must NEVER open a file/program unless you are 100% sure it is not infected. No matter how attractive the file is, where it came from, or what it promises you, you can never assume that a file is what it claims to be. For example, the Melissa virus reproduced through email and sent copies of itself to every one in the victim's address book. Because of this, relatives and friends of the victim were soon infected as well, because they assumed that the file was safe.
Your other defense is to use an updated virus protection program. If a program contains malicious code, or if the files on your computer match patterns created by a known virus, the software will alert you and will quarantine the infected file.
Unfortunately, antivirus software has its limitations. Because it compares the code in your files against an existing database of virus definitions, the protection is only as good as the last time you updated the database. Because new viruses are released onto the Internet every month, it is not long before your virus protection software is hopelessly outdated .
If you find out that you have been infected, there is hope. Most viruses will not destroy the infected device, because that would limit the lifespan of the virus itself. A malicious code writer often wants to infect as many computers as possible, which calls for stealth on the part of the virus. Thus, your goal is to detect the virus before it causes any harm.
The relationship between the client side and the server side of a Trojan is just like the relationship between a stereo remote control and the stereo unit itself. Instead of going to the stereo to adjust the station or CD track to which you are listening, you simply click on a button on your remote and the stereo reacts. The stereo unit has a device that listens for signals coming from the remote, much like the server side of a Trojan does. When you want to play a CD, you push a button on the remote, which sends a signal to the stereo unit. The remote does not actually change the CD, it merely sends the order to the stereo unit to change the CD. This is the basis for the client/server aspect of a Trojan. One main difference is that the server program will actually send a signal back to the client program to let the hacker know the command has been executed. Another difference is that the Trojan might allow for the transfer of files between the client and server.