One of the main reasons of destruction of information now is distribution of computer viruses.
The computer virus is a special computer program, as a rule, small by the sizes which at the start destroys or spoils the data which are storing on the computer. The computer virus can carry out the following harmful actions:
- removal or distortion of files;
- change (damage) of the table of placement of files on a disk which is responsible for integrity of data;
- contamination of random access memory and memory of a disk empty information;
- delay of operation of the computer or its full stop (lag).
The computer virus can "attribute" itself to other programs as speak, "to infect" them. Such "infection" leads to that computer viruses can independently extend and breed. Owing to what, the large number of computers can fail at the same time.
The program in which there is a virus, is called "infected". The mechanism of action of the infected program the following. When such program starts working, at some point management gets a virus which carries out harmful actions, and then infects other programs. After the virus will execute the harmful actions, it transfers control to that program in which is. And this program continues to work the same as usually. Thereby externally work of the infected program looks as well as not infected.
All actions of a virus are carried out quickly and imperceptibly. Therefore to the user of the personal computer happens very difficult to notice that in the computer there is a harmful work of a virus. However there is a number of signs on which it is possible to make the assumption of computer infection. The following concerns to them:
- programs cease to work or start working incorrectly (for example, "to hang", make the wrong calculations, to lose data, etc.);
- on the screen foreign messages, symbols, drawings, etc. are displayed;
- work on the computer is significantly slowed down or the computer hangs;
- there is a sudden loss of data on a disk;
- some files are spoiled, or they completely are removed etc.
At emergence of such signs it is necessary to inspect the computer on existence of viruses.
In order that the computer caught a virus, it is necessary that this virus got into the computer. The virus can get by means of portable devices which are used for data transmission between computers, via the local computer network of the organization, via other computer networks (for example, on the Internet).
However besides virus penetration on the computer there is one more condition computer infection. This condition consists in that on the computer the program containing a virus at least was once executed. Therefore direct infection of the computer with a virus can occur in one of the following cases:
- on the computer the infected program was executed;
- the computer was loaded from the diskette containing infected loading sector;
- on the computer the infected operating system was installed;
- on the computer the files containing in the body infected macroes were processed.
Despite possibility of infection of the computer a virus, it is necessary to know that all files of the computer can catch a virus not. The following components of file system of the computer are exposed to virus attack:
- executable files, i.e. files with the EXE, COM, BAT expansions;
- loading sector and the main loading sector of disks (i.e. sectors of magnetic disks which are used for loading on the computer of an operating system);
- drivers of devices and dynamic libraries (usually have the SYS and DLL expansion);
- graphic files, for example, with the JPG expansion;
- the files comprising macroes, for example, files with spreadsheets (XLS expansion), фалы with documents (DOC expansion).
Text files (with the TXT expansion), files with raster drawings (BMP expansion) and some other files, as a rule, don't catch computer viruses.
Viruses which after the start remain in random access memory are most dangerous and constantly infect computer files until it won't be switched off or rebooted. Those viruses which infect loading sectors of disks are also dangerous also. As if the loading sector of the winchester is infected, each time when loading computer this virus will be started and infect again new programs.
While the viruses, capable to put out of action the computer hardware are unknown, but there are viruses which can change the password to start of the computer and by that not to give the chance to start to work.a