A fairly standard way of doing this is using Audacity (
http://audacity.sourceforge.net/) and simply playing the Midi file using something like Windows Media Player and recording the result using Audacity. It works with most sound cards. Midi and Wav are inherently different formats - one describes how the sound is played (midi) and the other is essentially a recording of a sound. It's therefore not a "conversion" but much closer to playing and recording the midi.