A computer virus is a malicious software program that spreads by copying itself to other computers. It can corrupt or delete data on a computer. Viruses are often spread through email attachments, downloads, and removable media. There are several types of viruses including boot viruses, program viruses, macro viruses, and polymorphic viruses. Symptoms of a virus infection include programs not working properly, critical files being deleted, and the computer running slower. It is important to have antivirus software installed and keep it updated to protect computers from virus infections.

1. COMPUTER
SOFTWARE
VIRUSES
HARDWARE

3. What is Computer Virus..??
 A computer virus is a small software program that spreads from one
computer to another computer and that interferes with computer operation. A
computer virus may corrupt or delete data on a computer, use an e-mail program
to spread the virus to other computers, or even delete everything on the hard
disk.
 Computer viruses are most easily spread by attachments in e-mail
messages or by instant messaging messages. Therefore, you must never open an
e-mail attachment unless you know who sent the message or unless you are
expecting the e-mail attachment. Computer viruses can be disguised as
attachments of funny images, greeting cards, or audio and video files. Computer
viruses also spread by using downloads on the Internet. Computer viruses can be
hidden in pirated software or in other files or programs that you may download.

4. VIRUSES – TYPES AND EXAMPLES
• BOOT VIRUSES :-
These viruses infect disc boot records or master boot records in hard disks. They
replace the boot record program (which is responsible for loading the operating system in
memory) copying it elsewhere on the disk or overwriting it. Boot viruses load into memory
if the computer tries to read the disk while it is booting.
Examples: Form, Disk Killer, Michelangelo, and Stone virus
• PROGRAM VIRUSES:-
These infect executable program files, such as those with extensions like .BIN, .COM, .EXE,
.OVL, .DRV (driver) and .SYS (device driver). These programs are loaded in memory during
execution, taking the virus with them. The virus becomes active in memory, making copies
of itself and infecting files on disk.
Examples: Sunday, Cascade

5. •Multipartite viruses:
A hybrid of Boot and Program viruses. They infect program files and when
the infected program is executed, these viruses infect the boot record. When you
boot the computer next time the virus from the boot record loads in memory and
then starts infecting other program files on disk.
Examples: Invader, Flip, and Tequila
• Stealth viruses:
These viruses use certain techniques to avoid detection. They may either redirect the
disk head to read another sector instead of the one in which they reside or they may alter
the reading of the infected file’s size shown in the directory listing. For instance, the Whale
virus adds 9216 bytes to an infected file; then the virus subtracts the same number of
bytes (9216) from the size given in the directory.
Examples: Frodo, Joshi, Whale
• Polymorphic viruses:
A virus that can encrypt its code in different ways so that it appears differently in
each infection. These viruses are more difficult to detect.
Examples: Involuntary, Stimulate, Cascade, Phoenix, Evil, Proud, Virus 101

6. •Macro Viruses:-
A macro virus is a new type of computer virus that infects the macros within a document or
template. When you open a word processing or spreadsheet document, the macro virus is activated
and it infects the Normal template (Normal.dot)-a general purpose file that stores default document
formatting settings. Every document you open refers to the Normal template, and hence gets infected
with the macro virus. Since this virus attaches itself to documents, the infection can spread if such
documents are opened on other computers.
Examples: DMV, Nuclear, Word Concept.
• Active X: -
ActiveX and Java controls will soon be the scourge of computing. Most people do not
know how to control there web browser to enable or disable the various functions like
playing sound or video and so, by default, leave a nice big hole in the security by allowing
applets free run into there machine. There has been a lot of commotion behind this and
with the amount of power that JAVA imparts, things from the security angle seem a bit
gloom

7. COMPUTER TROJANS
COMPUTER TROJANS ARE SIMPLY MALICIOUS COMPUTER
PROGRAMS DISGUISED AS SOMETHING USEFUL.
THE MAJOR DIFFERENCE BETWEEN VIRUSES AND TROJANS IS THAT
VIRUSES REPRODUCE, WHILE A TROJAN IS JUST A ONE TIME
PROGRAM WHICH EXECUTES ITS PAYLOAD AS SOON AS THE TROJAN
IS EXECUTED
TROJANS ARE THE MOST COMMON WAY OF BRINGING A VIRUS
INTO A SYSTEM.
A CURRENT EXAMPLE OF A TROJAN IS A PROGRAM CALLED
PKZ300B.EXE WHICH DISGUISES ITSELF AS AN ARCHIVING UTILITY,
BUT WHEN RUN IT WILL DELETE YOUR ENTIRE HARD DRIVE.

8. COMPUTER WORMS
 Computer Worms are reproducing programs that run independantly and
travel across network connections. The main difference between viruses
and worms is the method in which they reproduce and spread. A virus is
dependant upon a host file or boot sector, and the transfer of files between
machines to spread, while a worm can run completely independently and
spread of its own will through network connections.
 An example of a worm is the famous internet worm of 1988: Overnight the
worm copied itself across the internet, infecting every Sun-3 and VAX
system with so many copies of itself that the systems were unusable.
Eventually several sites disconnected themselves from the internet to avoid
reinfection.

10. HOW DO COMPUTER VIRUSES SPREAD
Computer viruses usually spread in one of three
ways:-
• From Removable media.
• From Downloads off the Internet.
• From e-mail attachments.

11. Removable Disc
E-mail
Virus..

12. COMPUTER VIRUS SYMPTOMS
 Computer is infected with a malicious computer virus, it may begin to show a
series of Computer Virus symptoms that indicate that a virus is present.
 Computer viruses can be very harmful and damaging to your computer system,
root files, and various program files. A computer virus can also increase the risk of
hacker threats and attacks, and open the door to even more unauthorized
downloads and software installations.
 When a virus begins to spread, many program files can become unstable and
display warnings.
 The operating system may not launch properly, and the computer user may need
to reboot and restart the computer frequently to ensure all programs are starting
and working fine.
 Critical files may get deleted automatically, and this can happen periodically or all
at once. Error messages will become prevalent; it may become difficult to save
documents, and the computer may be running slower than usual.

13. SYMPTOMS OF WORMS AND TROJAN HORSE
 When a computer virus infects e-mail messages or infects other files on a
computer, you may notice the following symptoms:
 The infected file may make copies of itself. This behavior may use up all the free
space on the hard disk.
 A copy of the infected file may be sent to all the addresses in an e-mail address
list.
 The computer virus may reformat the hard disk. This behavior will delete files
and programs.
VIRUSES IN E-MAIL MESSAGES

14. A COMPUTER VIRUS CAN DAMAGE YOUR
SYSTEM
 A computer virus can do a lot of damage to your system before you even realize
that it happened. Leaving your computer unprotected is a primary reason why
most of the computer viruses get into your system.
 Different types of computer viruses that invade your system. Scum ware, Adware,
Spyware, Trojan Horses, computer viruses and Worms. Each one attacks your
system in a similar way.
 Computer Viruses: Quickly spreads from one computer to another computer. It
starts by attaching and loading itself onto the computer without your knowledge.
 A computer virus can be duplicated.

15. HOW TO REMOVE A COMPUTER VIRUS
Install and Update the antivirus software on the
computer. Then, perform a thorough scan of the
computer by using the antivirus software.
Install the latest updates from Microsoft Update on
the computer.
Download, install, and then run the Malicious
Software Removal Tool to remove existing viruses
on the computer. To download the Malicious
Software Removal Too

16. HOW TO PROTECT YOUR COMPUTER AGAINST
VIRUSES
On the computer, turn on the firewall.
Keep the computer operating system up-to-date.
Use updated antivirus software on the computer.

17. CONCLUSION..!!