In reality, the external computers, in particular, external infected computers are connected to the Internet. Based on this reasonable assumption, a new computer virus propagation model is established. Different from all the previous models, this model regards the external computers as a single compartment to study. Through a qualitative analysis, it is found that (1) this model possesses a unique (viral) equilibrium, and (2) this equilibrium is globally asymptotically stable. Further study shows that, by taking effective measures, the number of infected computers can be made below an acceptable threshold.