By taking into account the fact that, in general, a computer immediately possesses infectivity as soon as it is infected, a novel computer virus propagation model, known as the SLBS model, is established. It is proved that the dynamic behaviour of the model is determined by a threshold R0. Specifically, the virus-free equilibrium is globally asymptotically stable if R0≤ 1, whereas the virulent equilibrium is globally asymptotically stable if 1 < R0≤ 4. It is conjectured that the virulent equilibrium is also globally asymptotically stable if R0> 4. These results suggest some effective strategies for eradicating computer viruses distributed in the Internet.