Tips n Tricks: Computer

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What is a Computer Virus ? 
A potentially damaging computer programme capable of reproducing itself causing
great harm to files or other programs without permission or knowledge of the
user.  
 
Types
of viruses :- 
The different types of viruses are as follows- 
 
1) Boot Sector Virus
:- Boot sector viruses
infect either the master boot record of the hard disk or the floppy drive. The
boot record program responsible for the booting of operating system is replaced
by the virus. The virus either copies the master boot program to another part
of the hard disk or overwrites it. They infect a computer when it boots up or
when it accesses the infected floppy disk in the floppy drive. i.e. Once a
system is infected with a boot-sector virus, any non-write-protected disk
accessed by this system will become infected. 
 
Examples of boot- sector viruses are Michelangelo and Stoned. 
 
2) File or Program
Viruses :- Some files/programs,
when executed, load the virus in the memory and perform predefined functions to
infect the system. They infect program files with extensions like .EXE, .COM,
.BIN, .DRV and .SYS . 
 
Some common file viruses are Sunday, Cascade. 
 
3) Multipartite
Viruses :- A multipartite virus is
a computer virus that infects multiple different target platforms, and remains
recursively infective in each target. It attempts to attack both the boot
sector and the executable, or programs, files at the same time. When the virus
attaches to the boot sector, it will in turn affect the system’s files, and
when the virus attaches to the files, it will in turn infect the boot sector. 
This type of virus can re-infect a system over and over again if all parts of
the virus are not eradicated. 
 
Ghostball was the first multipartite virus, discovered by Fridrik Skulason in
October 1989. 
Other examples are Invader, Flip, etc. 
 
4) Stealth Viruses :- These viruses are stealthy in nature means it
uses various methods for hiding themselves to avoid detection. They sometimes
remove themselves from the memory temporarily to avoid detection by antivirus.
They are somewhat difficult to detect. When an antivirus program tries to
detect the virus, the stealth virus feeds the antivirus program a clean image
of the file or boot sector. 
 
5) Polymorphic Viruses
:- Polymorphic viruses
have the ability to mutate implying that they change the viral code known as
the signature each time they spread or infect. Thus an antivirus program which
is scanning for specific virus codes unable to detect it's presense. 
 
6) Macro Viruses :- A macro virus is a computer virus that
"infects" a Microsoft Word or similar application and causes a
sequence of actions to be performed automatically when the application is
started or something else triggers it. Macro viruses tend to be surprising but
relatively harmless.A macro virus is often spread as an e-mail virus.
Well-known examples are Concept Virus and Melissa Worm. 
 
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