The computer network is not only of benefit to 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 path of the virus spreading. The author 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 anti-virus field are introduced, and some countermeasures of the net computer users are discussed. The computer network is not only of benefit to 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 path of the virus spreading. The author 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 anti-virus field are introduced, and some countermeasures of the net computer users are discussed.