- Boot Sector Virus: replaces or implants itself in the boot sector - an area of the hard drive (or any other disk) accessed when you first turn on your computer. This kind of virus can prevent you from being able to boot your hard disk/computer.
- File Virus: infects applications. These executables then spread the virus by infecting associated documents and other applications whenever they're opened or run.
- Macro Virus: Written using a simplified macro programming language, these virusesaffect Microsoft Office applications, such as Word (.doc) and Excel (.xls). A document infected with a macro virus generally modifies a pre-existing, commonly used command (such as Save) to trigger its payload upon execution of that command.
- Multipartite Virus: infects both files and the boot sector - a double whammy that can
reinfect your system dozens of times before it's caught.
- Polymorphic Virus: changes code whenever it passes to another machine; in theory these
viruses should be more difficult for antivirus scanners to detect, but in practice they're
usually not that well written.
- Stealth Virus: hides its presence by making an infected file not appear infected, but
doesn't usually stand up to antivirus software.
Depending on the virus, some will perform more malicious deeds than others. Examples include deleting and renaming of files, scrambling contents of the entire hard drive (so you can’t access your data), or not letting the machine boot into Windows. Some viruses also slow down your machine, disable certain functions, or cause erratic system behavior and crashes.