This paper describes WMPI, the first full implementation of the Message Passing Interface standard (MPI) for clusters of Microsoft's Windows platforms (Win32). Its internal architecture and user interface, and some performance test results (for release v1.1), that evaluates how much of the total underlying system capacity for communication is delivered to the MPI based parallel applications, are presented. WMPI is based on MPICH, a portable implementation of the MPI standard for UNIX^® machines from the Argonne National Laboratory and, even when performance requisites cannot be satisfied, it is a useful tool for application developing, teaching and training. WMPI processes are also compatible with MPICH processes running on Unix workstations.