• What is a computer virus?
A computer virus is usually a program designed to replicate and spread, infecting as many computers without users aware of this. Viruses spread by attaching to other programs, EXE or COM files, and more recently, and Word documents, Excel, even HLP files, or some can infect the boot sector of the disk. When it starts running an infected file, or when you start the computer from a floppy disk or a virus, the virus is launched and running. Often, the virus remains resident in computer memory, to infect the next program launched in execution, or the next disk accessed.
What are dangerous viruses is their ability to perform actions on the computer. While some of these actions are annoying (such as displaying a message on a certain date or in response to a particular computer user action) and some annoying (such as reducing computer performance), there are viruses that can cause real disasters destroying data files, documents, or making the computer unusable.
The first viruses have emerged several decades ago, but have not spread globally known only after the appearance first PCs. In 1981 IBM put on, with giant mainframes that had brought him success, a computer “personnel” based on the new (then) processor produced by Intel Company, 8088. Its price was extremely high, but the product was a success. As operating system MS-DOS IBM bought the company Microsoft, which in turn wrote to the operating system CP / M. Early versions of DOS were very compact (only few tens of Kbps) and had no security protocol included.
5 years pass and reach the public in 1986 when the first reports appeared indicating viral entities IBM-PC. It was the Brain virus, a boot virus. Occur Antivirus programs designed to eliminate computer viruses. If the first antivirus programs were very simple then as viruses, programs today are true “masterpieces” of algorithms and code.
• How do viruses spread? 
Viruses can come from a variety of sources. Because a virus is executable code, it may be transmitted by all normal means of transmitting information between computers:
In a 1991 study commissioned by the company Dataquest National Computer Security Association in the United States, most viruses are transmitted by infected diskettes (87%).43% of infected diskettes responsible for introducing computer viruses on floppy disks were brought from home business. Almost three quarters (71%) of infections occurred in businesses with computer networks, increasing damage by rapid spread of viruses throughout the network. In network environments, the risk of virus infection is greatly increased. Seven percent (7%) of virus derived from files downloaded from various BBS (at that time the Internet was not the current spread). Other sources were infected floppy disks containing software demos and archived – about 6% of reported infections.
• What can viruses?
As I said earlier, some viruses are annoying, others can be very dangerous. If the happiest, viruses and reduce the size increase response speed, affecting the performance of your computer. Many seek only to spread viruses, does not affect your computer, so no harm intentionally. However, there is the possibility that benign viruses chance to interact with other software or even hardware and to slow or stop the system. Other viruses are more dangerous. They may alter or destroy data, or delete files and can reformat the hard drive.
• What are the symptoms of a virus system?
Those who are initiated into the field of computer viruses will probably not difficult to tell if a computer is suspected of being infected with a virus again. Viruses can spread freely as long as it remains undetected. For this reason, most viruses do not show their presence in the system. But antivirus software can detect the presence of such infections. There is, however, several viruses that make their presence felt in the system caused by side effects.
Some “symptoms” specific computer virus (the list is indicative, actual cases are more numerous and diverse)
V system files grow in length (e.g. 6.20 DOS command.com file is 54,619 bytes and a computer virus he may have, say with more than 1200 bytes, i.e. 55 819); 
Block V common – most viruses are extremely poorly written and very often the computer freezes. Viruses are also known as the most incompatible programs (except for multi-platform viruses, such as class “Concept” – viruses. Word doc);
V strange messages, songs or sounds suspicious speaker. Many viruses make their presence announced by such effects;
V data destruction is another result of viruses. Sudden disappearance of a file or file system errors are very common;
V slow disk access is caused by some stealth viruses which are interposed between system software and disk access;
V when you press CTRL + ALT + DEL Boot the computer instantly without going through the POST screen (Power On, Self Test)
V the chkdsk command most executable programs are reported as having an incorrect length: the effect of stealth viruses;
V memory size displayed by specialized programs is less than 640Kb. Sometimes this effect is caused by some memory managers without talking about a virus, but usually indicates the presence of a virus;
V Self-Check program reports that have been modified;
V not start Windows or reports that disk access is via the BIOS;
V changes the file timestamp;
V heavier loading of programs;
V slow computer operation;
V bad sectors on diskettes.
