Types of Viruses

Several types of viruses exist, but the popular ones are file viruses, macro (data file) viruses, and boot sector viruses. Each type differs slightly in the way it works and how it infects your system. Many viruses attack popular applications such as Microsoft Word, Excel, and PowerPoint; they are easy to use and it’s easy to create a virus for them. Because writing a unique virus is considered a challenge to a bored programmer, viruses are becoming more and more complex and harder to eradicate.
File Viruses
A file virus attacks executable application and system program files, such as those ending in .COM, .EXE, and .DLL. Most of these types of viruses replace some or all of the program code with their own. Only once the file is executed can the virus cause its damage. This includes loading itself into memory and waiting to infect other executables, further propagating its potentially destructive effects throughout a system or network. Examples of file viruses are Jerusalem and Nimda (although Nimda is usually seen as an Internet worm) may also infect common Windows files, as well as files with extensions such as .HTML, .HTM, and .ASP.
Macro Viruses
A macro is a script of commonly enacted commands that are used to automatically perform operations without a user’s intervention. Macro viruses use the Visual Basic macro scripting language to perform malicious or mischievous functions in data files created with Microsoft Office products, for example. Macro viruses are among the most harmless (but also the most annoying). Since macros are easy to write, macro viruses are among the most common viruses and are frequently found in Microsoft Word and PowerPoint. They affect the file you are working on. For example, you might be unable to save the file even though the Save function is working, or you might be unable to open a new document—you can only open a template. These viruses will not crash your system, but they are annoying. Cap and Cap A are examples of macro viruses.
Boot Sector Viruses
Boot sector viruses get into the master boot record. This is track one, sector one on your hard disk, and no applications are supposed to reside there. The computer at bootup checks this section to find a pointer for the operating system. If you have a multi-operating-system boot between various versions or instances of Windows, for example, this is where the pointers are stored. A boot sector virus will overwrite the boot sector, thereby making it look as if there is no pointer to your operating system. When you power up the computer, you will see a Missing Operating System or Hard Disk Not Found error message. Monkey B, Michelangelo, Stoned, and Stealth Boot are examples of boot sector viruses.