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Computer viruses attack the software of a computer, not the hardware. The basic definition of a virus is a program that copies itself. Most common viruses are small programs that need to use a legitimate program or file in order to run. Most viruses today are written with malicious intent; as a result, they can cause damage to programs or data. Because the virus code must be run to have any effect, the files that the computer treats as pure data, such as .txt files, are safe. For example, reading a plain-text e-mail message won't infect your computer with a virus.

In order to run, the virus code has to be written in an executable form, such as an .exe program file. HTML-format and RTF-format e-mail messages or Web pages that contain Microsoft ActiveX® controls or scripts, such as Microsoft Visual Basic® Scripting Edition (VBScript), Microsoft JScript®, or other types of embedded program code, can harbor virus programs that run when you open a message or view a Web page. Opening e-mail attachments can also infect your computer with viruses. Some malicious files can be constructed to take advantage of known vulnerabilities in the software in order to run destructive code. Microsoft recommends that you protect your computer by applying all updates and patches for the software on your computer.

Macro viruses

A macro is a series of commands and instructions that are grouped together as a single command to accomplish a task automatically. If you perform a task repeatedly in an application, you can automate the task by using a macro. You can store macros in documents, worksheets, or templates, which makes them available whenever a new file based on that template is created. For example, Microsoft Word stores user-recorded macros in the Normal template (Normal.dot) by default, so that they are available for use with every Word document. When you open the Word document, the macro runs. A macro virus is a virus program written in Microsoft Visual Basic® for Applications, the same macro language used in legitimate macros. A macro virus can also run automatically when you open a document unless there are safeguards in place. Most Microsoft Office programs display a confirmation dialog box when you choose to open a document that contains macros.

How do viruses spread?

The Melissa virus in March 1999 spread in the form of an e-mail message with an attached Word document that contained a macro virus. Anyone who opened the attachment triggered the virus. The virus would then send the document (and therefore itself) in an e-mail message to the first 50 people in the person's address book. The e-mail message contained a friendly note that included the person's name, so the recipient would open the document, thinking it was harmless. The virus would then create 50 new messages from the recipient's address book. As a result, the Melissa virus was the fastest-spreading virus ever seen and forced a number of large companies to shut down their e-mail systems.

The important thing to remember is that just because an e-mail message appears to come from someone you trust, this does not mean the file is safe or that the sender had anything to do with it. Also, keep in mind that when you share files with another user, the attached macro or script is included with the file. Therefore, be careful when you share files, and scan the files with an antivirus program before you open them. You can choose from many antivirus application vendors.

Important Before you scan Outlook e-mail messages, check with the antivirus program vendor to make sure it is compatible with Outlook. Some antivirus programs can cause problems with Outlook.