Computer virus

A computer virus is a computer program that can copy itself and infect a computer without the permission or knowledge of the owner. The term "virus" is also commonly but erroneously used to refer to other types of malware, adware and spyware programs that do not have the reproductive ability. A true virus can only spread from one computer to another (in some form of executable code) when its host is taken to the target computer; for instance because a user sent it over a network or the Internet, or carried it on a removable medium such as a floppy disk, CD, DVD, or USB drive. Viruses can increase their chances of spreading to other computers by infecting files on a network file system or a file system that is accessed by another computer.^[1][2]

The term "computer virus" is sometimes used as a catch-all phrase to include all types of malware. Malware includes computer viruses, worms, trojan horses, most rootkits, spyware, dishonest adware, crimeware and other malicious and unwanted software), including true viruses.

Viruses are sometimes confused with computer worms and Trojan horses, which are technically different. A worm can use security vulnerabilities to spread itself to other computers without needing to be transferred as part of a host, and a Trojan horse is a program that appears harmless but has a hidden agenda. Worms and Trojans, like viruses, may cause harm to either a computer system's hosted data, functional performance, or networking throughput, when they are executed. Some viruses and other malware have symptoms noticeable to the computer user, but most are surreptitious. This makes it hard for the average user to notice, find and disable and is why specialist anti-virus programs are now commonplace.

Most personal computers are now connected to the Internet and to local area networks, facilitating the spread of malicious code. Today's viruses may also take advantage of network services such as the World Wide Web, e-mail, Instant Messaging and file sharing systems to spread, blurring the line between viruses and worms. Furthermore, some sources use an alternative terminology in which a virus is any form of self-replicating malware.

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.

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.