Computer viruses: description, prevention, and recovery

What is a 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.


Symptoms of a computer virus

If you suspect or confirm that your computer is infected with a
computer virus, obtain the current antivirus software. The following
are some primary indicators that a computer may be infected:

• The computer runs slower than usual.
  
• The computer stops responding, or it locks up frequently.
  
• The computer crashes, and then it restarts every few minutes.
  
• The computer restarts on its own. Additionally, the computer does not run as usual.
  
• Applications on the computer do not work correctly.
  
• Disks or disk drives are inaccessible.
  
• You cannot print items correctly.
  
• You see unusual error messages.
  
• You see distorted menus and dialog boxes.
  
• There is a double extension on an attachment that you recently opened, such as a .jpg, .vbs, .gif, or .exe. extension.
  
• An antivirus program is disabled for no reason. Additionally, the antivirus program cannot be restarted.
  
• An antivirus program cannot be installed on the computer, or the antivirus program will not run.
  
• New
  icons appear on the desktop that you did not put there, or the icons
  are not associated with any recently installed programs.
  
• Strange sounds or music plays from the speakers unexpectedly.
  
• A program disappears from the computer even though you did not intentionally remove the program.
  

Note

These are common signs of infection. However, these signs may also be
caused by hardware or software problems that have nothing to do with a
computer virus. Unless you run the Microsoft Malicious Software Removal
Tool, and then you install industry-standard, up-to-date antivirus
software on your computer, you cannot be certain whether a computer is
infected with a computer virus or not.


Symptoms of worms and trojan horse viruses in e-mail messages

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.
  
• The
  computer virus may install hidden programs, such as pirated software.
  This pirated software may then be distributed and sold from the
  computer.
  
• The computer virus may reduce security. This could enable intruders to remotely access the computer or the network.
  
• You
  receive an e-mail message that has a strange attachment. When you open
  the attachment, dialog boxes appear, or a sudden degradation in system
  performance occurs.
  
• Someone tells you that they have recently
  received e-mail messages from you that contained attached files that
  you did not send. The files that are attached to the e-mail messages
  have extensions such as .exe, .bat, .scr, and .vbs extensions.
  

Symptoms that may be the result of ordinary Windows functions

A computer virus infection may cause the following problems:

• Windows
  does not start even though you have not made any system changes or even
  though you have not installed or removed any programs.
  
• There is
  frequent modem activity. If you have an external modem, you may notice
  the lights blinking frequently when the modem is not being used. You
  may be unknowingly supplying pirated software.
  
• Windows does not
  start because certain important system files are missing. Additionally,
  you receive an error message that lists the missing files.
  
• The
  computer sometimes starts as expected. However, at other times, the
  computer stops responding before the desktop icons and the taskbar
  appear.
  
• The computer runs very slowly. Additionally, the computer takes longer than expected to start.
  
• You receive out-of-memory error messages even though the computer has sufficient RAM.
  
• New programs are installed incorrectly.
  
• Windows spontaneously restarts unexpectedly.
  
• Programs
  that used to run stop responding frequently. Even if you remove and
  reinstall the programs, the issue continues to occur.
  
• A disk utility such as Scandisk reports multiple serious disk errors.
  
• A partition disappears.
  
• The computer always stops responding when you try to use Microsoft Office products.
  
• You cannot start Windows Task Manager.
  
• Antivirus software indicates that a computer virus is present.
  

Note

These problems may also occur because of ordinary Windows functions or
problems in Windows that are not caused by a computer virus.


How to remove a computer virus

Even for an expert, removing a computer virus can be a difficult task
without the help of computer virus removal tools. Some computer viruses
and other unwanted software, such as spyware, even reinstall themselves
after the viruses have been detected and removed. Fortunately, by
updating the computer and by using antivirus tools, you can help
permanently remove unwanted software.

To remove a computer virus, follow these steps:

1. Install the latest updates from Microsoft Update on the computer.
   
2. Update the antivirus software on the computer. Then, perform a thorough scan of the computer by using the antivirus software.
   
3. 
   Download, install, and then run the Microsoft Malicious Software
   Removal Tool to remove existing viruses on the computer. To download
   the Malicious Software Removal Tool, visit the following Microsoft Web
   site: [You must be registered and logged in to see this link.]
   (http://www.microsoft.com/security/malwareremove/default.mspx)
   

For more information about how to remove a computer virus, visit the following Microsoft Web site: [You must be registered and logged in to see this link.]
(http://www.microsoft.com/protect/computer/viruses/remove.mspx)



How to protect your computer against viruses

To protect your computer against viruses, follow these steps:

1. On the computer, turn on the firewall.
   
2. Keep the computer operating system up-to-date.
   
3. Use updated antivirus software on the computer.
   
4. Use updated antispyware software on the computer.
   

For more information about how to protect a computer against viruses, visit the following Microsoft Web site: [You must be registered and logged in to see this link.]
(http://www.microsoft.com/protect/computer/default.mspx)


ORIGINAL LINK

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