The greater threat to computer is a computer virus,a program that causes a computer system to behave in unexpected and undesirable ways.Some signs of a virus include inexplicable loss of free memory,unusually long times for program loading or execution,changes in file or program size,print routines that stop working ,computers”freezing up”‘strange beeps or messages,computer reboot in the middle of a process,and corrupted files.When a virus is executed,it makes copies of itself and spreads from one system to another through network and diskettes.