(1) Computer viruses are small software programs that are designed to spread from one computer to another and to interfere with computer operation. Internet is the most common media for the spread of viruses. PCs and Compact Disks are susceptible to virus attacks, but such an infection can be removed with the help of anti-virus programs. Similarly, Firewalls are frequently used to prevent unauthorized Internet users from accessing private networks connected to the Internet, especially intranets.