What is a Virus?
A virus is a piece of self-replicating program code. It is a software program which is designed to copy itself.
Boot sector viruses infect the boot sector of floppy disks and the partition sector or the boot sector of hard disks when the PC is booted. Executable file viruses infect program files on local drives or network drives. Macro viruses infect the macros within files such as Word documents and Excel spreadsheets.
Most viruses try to conceal themselves from users and from anti-virus programs. Some viruses contain a "payload". This may be anything from a screen display (such as a fireworks display), a message, or something designed to intentionally damage data files or file systems. Not all viruses contain a payload. If the virus does contain a payload, there is usually a trigger which causes the virus to deliver its payload. The trigger may be a particular system date, the number of re-boots, the number of floppy disks infected or something else which software can be designed to do.
Viruses may also produce unintended consequences and they may make a computer's operating system unstable or prevent other software or parts of your computer from working properly.