h**p://www.stardate.bc.ca/eawpatches/html/default.htm
TiMIDIty !
You want the high quality (Cspline) command line version and the GUI front end, in preference to the "realtime" command line or Win32 versions, plus the fairly large patch set.
The alternative is to "play through" using a wavetable soundcard, and simultaneously record from "Midi Synth" or "What U Hear" or "Stereo Mix".
A direct midi to wav conversion may be a little "dry", so you may prefer the play-through approch with a TINY bit of reverb/chorus or other effect applied - if you have an SB Live, GENTLE use of EAX effects may be appropriate.