Computer is an electronic machine that can do a lot of miracles for the man, which was created by man. Though it is created by man, it can work faster and more accurate than man. It can resolve the problems, which even man could not resolve. Initially when computers were invented, the size of its machine was bigger than the size of a 32 inch T.V., which consumed more space. Then as days and years passed by, the human brain invented computers to a smaller size which can be easily shifted from one place to another and user-friendly.\r\nComputers can help us do calculations, store information, files and documents, etc. As we have locks for lockers, even computer has locking system. If we want to prevent any secret information from falling into the hands of others, such kind of information can be stored in a computer with security locks and passwords. Not only for doing such kind of works, but computers are also used for playing games, listening to songs, watching videos and surfing internet. Nowadays, in many houses we could see computers. To prevent our computer from the threats of viruses, proper security system has to be installed in it.