A great number of computer problems can be caused by computer
viruses. Effects of a virus could be minor or severe (fatal), and they
might be predictable or sporadic. Unfortunately, diagnosing and
removing viruses can be difficult, and with the increased information
exchange brought on by access to the Internet, viruses are becoming
more and more prevalent.
Computer viruses are not caused by corrupted files or internal OS or
application flaws. Rather, they are intentionally created programs, the
purpose of which is to cause some effect in the computer and replicate
themselves to be passed on to other computers. The effect that a virus
has on a computer is called payload. A virus payload could be
nondestructive to the computer, meaning that it could merely display a
particular message, run a video clip, or change the display colors.
However, if a payload is destructive, it can delete files, close running
applications, or destroy a drive’s master boot record.

Virus

Virus Types

Many types of computer infestations are actually not viruses at all. A true
virus is a piece of code that attaches itself to an executable file and is not
activated until the executable file is launched.
1. Worm is a
program in itself and does not need to attach itself to a legitimate application
in order to run. Viruses are typically more common than worms.
Viruses can be categorized by where they hide themselves.

2. File viruses hide themselves in executable
files. When the executable file is run, the virus is activated.

3. Macro virus attach themselves to
portions of applications and disguise themselves as macros. A macro is
simply an automated process within an application, such as reading and
automatically updating a date field or searching for and formatting specified
text.

4. Boot sector virus hides itself
in the MBR and is activated during startup when the MBR is located and
initialized
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