The common definition for a computer virus is a program that can copy itself in order to infect a computer. While the term "virus" is often used to refer to other types of malware (such as adware and spyware programs), a virus is only capable of being spread from one computer to another in some form of executable code when its host is taken to the target computer. For example, when a user sends a virus attachment via email, or if an infected file is on a disk or flash drive, then the virus can be spread.
Common File Extensions
The classic computer virus arrives as an executable file, ending in .exe, or a Visual Basic file, which ends in .vb*. If you happen to receive an email with this type of file attached, you should always exercise extreme caution before opening it.
Other File Extensions
There are also many less common file extensions that can be used by a virus to infect a computer. Some of these include:
ADE--Microsoft Access Project Extension ADP--Microsoft Access Project BAS--Visual Basic Class Module BAT--Batch File CHM--Compiled HTML Help File CMD--Windows NT Command Script COM--MS-DOS Application CPL--Control Panel Extension CRT--Security Certificate DLL--Dynamic Link Library DO--Word Documents and Templates HLP--Windows Help File HTA--HTML Applications INF--Setup Information File INS, ISP--Internet Communication Settings JS--JScript File JSE--JScript Encoded Script File LNK--Shortcut MD--Microsoft Access Applications and Databases MSC--Microsoft Common Console Document MSI, MSP--Windows Installer MST--Visual Test Source File OCX--ActiveX Objects PCD--Photo CD Image PIF--Shortcut to MS-DOS Program POT, PPT--PowerPoint Files REG--Registration Entries SCR--Screen Saver SCT--Windows Script Component SHB--Document Shortcut File SHS--Shell Scrap Object SYS--System Config/Driver URL--Internet Shortcut (Uniform Resource Locator) WS--Windows Scripts XL--Excel Files and Templates
Make File Extensions Visible
One of the primary ways to protect yourself from computer viruses is to know what files you're opening before you open them. In Windows Explorer, you can open your folder options and select the option to always display file extensions.
The majority of computer viruses are transmitted through email or instant messaging. A good rule of thumb is to never open a file unless you know exactly what it is. If you get an email with an unexpected attachment, you should not open it until you verify that it's legitimate and safe.
The Final Word on Safety
Of course, your computer should always be running anti-virus software with the latest definitions installed. You should also have a firewall, and keep your operating system updated with any necessary service packs or security patches.