The aim of this work is to describe a stochastic epidemic model for computer viruses, the SIRA model, with the help of a continuous time Markov chain. It is a modification of the stochastic SIR model and it considers antidotal computers that are equipped with totally effective anti-virus. In the stochastic SIRA model the infection becomes extinct with probability one, regardless of the parameters of the model. Under the assumption that the extinction has not occurred yet, it is interesting to study the number of infected computers, which is done in terms of the quasi-stationary distribution. Other descriptors are also analyzed, such as the extinction time, the number of cases of infections until the eradication of the virus and the number of infections during a period of time. Finally, some numerical results illustrate the analysis of these descriptors.