title: GUI freezes after os.system call
body: <p>My problem is with os.system. Until it finishes formatting, GUI freezes, and I can't fix it.</p>
  
  <pre><code>class ImageDialog(QtGui.QMainWindow):
      def __init__(self):
          QtGui.QDialog.__init__(self)
          self.ui = uic.loadUi("Recursos/main.ui",self) 
          self.connect(self.ui.proteger_Button, QtCore.SIGNAL("clicked()"),self,
                       QtCore.SLOT("protec()"))
  
      @QtCore.pyqtSlot()
      def protec(self):
          self.USB = "G:"
          comando = "format %s /fs:ntfs /q /v:test /y" %(self.USB)
          os.system(comando)`
  
  if __name__ == '__main__':
  
      app = QtGui.QApplication(sys.argv)
  
      window = ImageDialog()
      window.show()
  
      sys.exit(app.exec_())       
  </code></pre>