You can try using BitVoicer (http://www.bitsophia.com/BitVoicer.aspx) for the speech recognition and, based on what it sends to your Arduino, you can use a text-to-speech (synthesizer) board to provide answers. BitVoicer's Voice Schemas can handle hundreds of recognition possibilities, so if you do a good job with the Voice Schema, it might be possible to handle simple dialogs.