A computer virus is a program that can duplicate itself and infect computers without the authorization or knowledge of the owners.
It spreads from one computer to another (in the form of executable program) when the infected program moves to the target computer; for example a user may transmit over a network or carry it on a removable storage medium, such as a USB drive, floppy disk, CD or a DVD.
To make their own multiple copies, computer viruses join themselves to executable files that are part of genuine programs. When a user accesses the infected program, the code of the virus gets executed with it.
Viruses may be categorized into two varieties depending on their behavior upon execution.
Nonresident viruses: These viruses locate files to be infected, infect them, and then transfer control to them. Nonresident viruses are generally made of two parts: a finder module and a replication module. The finder module searches for new files that can be infected. When it finds a new executable file, it references the replication module to infect the file.
Resident viruses: These viruses consist of a replication module which is similar to that of the nonresident viruses. However, there is no finder module. These viruses place the replication module into the computer memory when they are executed for the first time. This ensures that the module gets executed every time the operating system performs a certain action. For example, the replication module can be executed, each time the operating system opens a file. In such a scenario the virus infects every program file that opens on your computer.
Viruses typically cause damage to your program files. Some viruses access address books and send malicious E-mails to your contacts. There are others that interfere with your computer's operation. Even more serious affects of virus infection may be hardware damage and operating system crashes.
Signs of Infection
The most common signs of virus infection on your computer include:
* Your computer runs slower than usual.
* Your computer hangs or stops responding very often.
* Your computer restarts without any command and then does not succeed to run normally.
* Your computer crashes and restarts on its own after every few minutes.
* You cannot access disks and disk drives.
* You observe twisted menus and dialog boxes.
* Programs disappear from your computer even though you do not remove them.
Most personal computers are now connected to the Internet and to local area networks, facilitating the spread of the computer viruses. These viruses take full advantage of network services such as the E-mails, instant messaging and file sharing systems to spread.
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