We consider the simulation of transient performance measures of high reliable fault-tolerant
computer systems. The most widely used mathematical tools to model the behavior of these
systems are Markov processes. Here, we deal basically with the simulation of the mean time to
failure (MTTF) and the reliability, R(t), of the system at time t. Some variance reduction
techniques are used to reduce the simulation time. We will combine two of these techniques:
Importance Sampling and Conditioning Technique. The resulting hybrid algorithm performs significant
reduction of simulation time and gives stables estimations.