The computer network is not only of benefit to the people, but also helpful for the spreading of viruses. The net-viruses spread more widely and rapidly than the traditional viruses, and network has become the major way of the virus spreading. This article analyzes the mode of the net-virus spreading, including E-mail spreading, positive scanning spreading and through-server spreading. Then some defense strategies in the antivirus field are introduced, and some countermeasures of the net computer users are discussed.