A computer virus is a software program designed to replicate and spread on its own and to affect your computer by altering the way it works without your knowledge or permission. Computer viruses spread by attaching themselves to another program (such as your word processing or spreadsheet programs) or to the boot sector of a diskette. When an infected file is executed, or the computer is started from an infected disk, the virus itself is executed.

Viruses are written with 2 separate purposes. The first purpose is to spreads itself from one file to another without knowledge. The second purpose is to fulfill the function contemplated by its creator called a payload.

The payload may be to damage some part of your computer like erasing a disk or corrupting your programs. As well, many viruses may not manifest themselves until a later date at which point they may simply display a message or may more detrimentally delete files. A virus that interferes with the functioning of your computer is referred to as a malicious virus and one that is designed to do no real damage to your computer is referred to as a benign virus.

There are many kinds of viruses. Viruses can attach themselves to program files (i.e., EXE or COM files) and are executed when you run the program.

Viruses also infect system (boot) sectors. Every hard disk and floppy drive contains a boot sector which contains specific information about the formatting of the disk and the data stored there. The boot sector also contains a small program called the boot program. The boot program gets infected by viruses. You get a boot sector virus by leaving an infected diskette in a drive and rebooting the machine. When the boot sector program is read and executed, the virus goes into memory and infects your hard drive.

A third virus type is the macro virus which are executed when you view a Microsoft Word document that contains the virus. It's even possible to write viruses for batch files, word processors, or spreadsheet macros.

Finally you can have viruses that are a combination of the viruses listed above.

While most viruses are harmless, many are not and therefor taking proper care to avoid having your computer infected is worthwhile especially since the malicious viruses can erase a disk or files.