Computer Viruses

Computer Viruses

A computer virus is a relatively small
program that attaches itself to data and program files before it delivers
its malicious act. There are many distinct types of viruses and each
one has a unique characteristic. Viruses are broken up into to main classes,
file infectors and system or boot-record infectors.

A file infector attaches to ordinary programs
usually affecting .COM and .EXE files. These two files are subject
to infection though some can infect any program from which execution is
requested. On the other spectrum, system or boot-record infectors
infect executable codes found in certain system areas on a disk, which
are not ordinary files.

Not only are there two main classes of
viruses, but there are also many different types. A virus can be
classified as a boot sector virus, a program virus, a trojan horse virus,
a macro virus, a memory resident virus, a stealth virus, or a polymorphic

A boot sector virus replaces the startup
code on a disk with a modified version, which is loaded into the memory
when the computer is started.

Program viruses attach to legitimate program
files. After attachment the program file will then intercept the
programís execution and take control of the computer system.

Trojan horse viruses are also legitimate
programs that are advertised as something desirable, but are actually something
that is undesirable. It performs an unauthorized destructive act
on your computer when executed. They received their name from the
mythical Trojan Horse that was delivered to the gates of the city Troy.

Macro viruses are written in macro language
of specific computer programs, such as Word or Excel. These viruses
infect files when a program document is accessed, or triggered by user
actions, such as certain keystrokes or menu choices.

Memory resident viruses hide in memory
and are difficult to detect. When the virus is in memory it will
infect any uninfected file that is executed.

A stealth virus hides their presence.

They can infect a program by adding bytes to the infected file. It
then subtracts the directory entry of the infected file by the same number
of bytes. This gives the impression that the fileís size has not
changed, therefore, antiviral programs can not detect them through these

Polymorphic viruses modify their appearance
and scramble their code periodically. This allows them to replicate
so that none of its copies are the same.

These viruses also all have different characteristics.

There are three main characteristics that a virus may have. This
includes a time bomb, a logic bomb, worm characteristics. A time
bomb is characterized as a virus that stops duplicating and becomes active
when a certain date registers on the computer systemís clock. On
the other hand, a logic bomb is triggered when a particular event occurs
and they are activated by the appearance or disappearance of creation data.

A virus can also be characterized as a worm. A worm is a virus-like
program that creates copies of itself continuously until the computer system
is overloaded and stops working. As seen here there are many different
characteristics of viruses. It has been said that more than fifteen
new viruses are created each day. Equip your computer system with
a reliable anti-virus software program to safe guard your system from computer