Since the coming of computer era, viruses have existed not just to test its vulnerability and when worse comes to worst, can have a devastating effect to a number of interconnected computer systems. Computer viruses defy overnight sensation and some of the known viruses like Mydoom crippled a quarter million of computers in 2004, the Melissa virus that infiltrated even big companies even Microsoft to turn off their e-mail systems and lastly the ILOVEYOU virus that went as far as the Pentagon. Then there was a worm called Storm in 2007 that affected almost 50 million computers. The difference between a worm and a virus is that a worm is a small piece of software that uses computer networks that replicates itself while a virus really works on a real program. So far the hardest one to deal with nowadays is the Trojan horse. This virus can never replicate but it can delete even your hard disk.