1. A software program capable of reproducing itself and usually capable of causing great harm to files or other programs on the same computer.
2. It must execute itself and replicate itself.
3. 1.File infector Viruses
2.Boot sector viruses
3.Master boot record viruses
4. Trojan Horses are impostors--files that claim to be something desirable but, in fact, are malicious
5.Worms are programs that replicate themselves from system to system without the use of a host file.
6. Hardware problems, computers beeps at starup no screen display, computer does not regidter 640 k of conventional memory, microsoft word and word warns you that a document contains a macro, and not able to open a particular documents.