Using multiprocessing on Windows can be different; please read http://docs.python.org/library/multiprocessing.html#windows especially the part named "Safe importing of main module".

On Windows, fork() does not exist, so a new interpreter must be started, which will import the current module; this must not start the test suite again!
Adding "if __name__ == '__main__'" somewhere is probably the solution.
If not, you should move the target function to another module.