What exactly is a computer virus? I have been asked this question many times and thought many of you too may be curious. Computer viruses are so named because of their similarities with biological viruses. Like the influenza virus which comes in many strains, computer viruses come in many forms as well. A single computer virus code like Nimda can have many variations such as Nimda.A, Nimda.E, Nimda.R, Nimda@mm, W32.Nimda, W32.Nimda.A and WormI.Nimda. The one outstanding feature of any virus is that its main goal is to reproduce itself. Some viruses do destroy data, but, this behavior is not the basis for classifying a piece of code as a computer virus. Some viruses are written to be as small as possible, and do not waste code with damage routines. Other viruses use large amounts of code so that the few lines of code that are used to replicate the virus are not noticed. The term virus was given to this type of malicious code due to its inherent ability to reproduce itself. Therefore, even if a piece of code does nothing harmful to the system but keeps on making copies of itself then it is considered to be a computer virus.

Please note that these elements are part of most viruses, but the level of adaptation may differ from species to species. There are essentially three parts of a computer virus:

The Replicator – The Replicator’s job is to ensure the survival of the virus on a system. Most successful viruses do this by not inflicting damage on the system but by appending themselves to legitimate programs in the machine. Each time the program is run the virus ‘wakes up’ and starts to reproduce. As stated earlier, this is the most important part of the virus code.

The Concealer – This part of the virus attempts to hide the virus. There are a variety of ways to accomplish this, but the main goal is to avoid detection by antivirus software. Antivirus software must be updated regularly to maintain its database of virus signatures. A virus signature is a set of characteristics that uniquely identifies a specific virus. Today’s viruses use various advanced techniques to stop being caught from Antivirus software.

The Payload – The payload of a virus can be practically anything. If a virus is going to have a long life then any damage it causes must either be very slight, or should not take place for a long period after infection. If an obvious payload gets delivered soon after infection then the user will notice that a problem exists and will quickly go virus hunting.

The Symptoms of Infection

Since many viruses release their payload slowly over time or wait a period of time before releasing the payload, the symptoms of a viral infection may appear gradually or can be confused with other computer issues.

The following symptoms are frequently caused by or associated with a virus:

• You received 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.
• There is a double extension on an attachment that you recently opened, such as .jpg.vbs or .gif.exe.
• An antivirus program is disabled for no reason and it cannot be restarted.
• An antivirus program cannot be installed on the computer or it will not run.
• Strange dialog boxes or message boxes appear onscreen.
• New icons appear on the desktop that you did not put there, or are not associated with any recently installed programs.
• Strange sounds or music plays from the speakers unexpectedly.
• A program disappears from the computer, but you did not intentionally remove it.

A virus infection may also cause the following symptoms, but these symptoms may also be the result of ordinary Windows functions, or problems in Windows that are not caused by a virus.

• Windows will not start at all, even though you have not made any system changes, and you have not installed or removed any programs.
• Windows will not start because certain critical system files are missing, and then you receive an error message that lists the missing files.
• The computer sometimes starts as expected, but at other times it stops responding before the desktop icons and taskbar appear.
• The computer runs very slowly, and it takes a long time to start.
• You receive out-of-memory error messages even though your computer has much RAM.
• New programs do not install correctly.
• Windows spontaneously restarts unexpectedly.
• Programs that used to run stop responding frequently. If you try to remove and reinstall the software, the issue continues to occur.
• A disk utility such as Scandisk reports multiple serious disk errors.
• Your computer always stops responding when you try to use Microsoft Office products.
• You cannot start Windows Task Manager.
• Antivirus software indicates that a virus is present.