Viruses are those malicious programmes that get into the computer’s operating system and performs certain operations that harm the computer or slows down the computer speed. Viruses are basically programmes that act like bots. Botnets can be used to send spam and attack other Web sites, but they need to communicate with a host computer to receive its own instructions. One such deadly botnet that is creating serious problems to computer worldwide is the conficker. Security researchers say that conficker has infected millions of computers worldwide. Conficker takes advantage of a vulnerability in Microsoft's software and has infected at least 3 million computers, making it one of the most severe viruses in recent years. Conficker affects the computer in two ways: it tries to visit a Web site and pick up instructions or it can receive a file over its custom-built encrypted network.