Can’t seem to find a free one 
From ZDNet: How do I convert MIDI files to WAV files?
An excerpt:
Never say never, evilhanz:
Not free, but at $14.95 it is cheap, and there is a downloadable demo.
I do not know why you would want to do this since a MIDI file is much smaller and has other advantages. But if you want to do it you can just play it in your computer and record to a wav file while it plays. I have done it ocassionally for someone whose computer could not play midis.
No not never. The easiest method is to route your synthesizer output to audio input and record the resulting music to wav files with your built-in recorder. This can either be done by configuring a high-end soundcard, or routing the output with cables. The author of the article whose link I posted seems to think programs like the one you mentioned (and the one the poster is looking for) are of poor quality. I guess the only way to be sure is to try the demo.
Try here:
http://www.polyhedric.com/software/
You will want the MIDInightExpress. It is free. Since it is a software wavetable synthesizer, I expect you will find it sounds better than the synthesizer built into your sound card (unless your soundcard has a wavetable synthesizer.) I have a couple of MIDI players on my system at home. One uses the soundcard synthesizer and sounds cheesy. The other is a software wavetable synthesizer, and sounds great. I use the wavetable synthesizer when I want to hear good playback of a MIDI file, and the other when the computer is running under a heeavy load (compiling a program or something in the background.) The software synthesizer uses a lot of processor time.
I have a Creative Soundblaster Live! Can anyone tell me how to route my synthesiser correctly so that I can change the output to input as posted above? And I want to be able to put MIDI on CDs, so I would need to convert them
Thanks
the easiest way to do this is to simply run the
windows mixer (that little speaker in the bottom
corner), Then select ‘Options’ and ‘recording’
then click OK. remove the checkmarks from everything
but MIDI.
then use your WAV software to simply record the
MIDI as it plays…
Your SBLive card should have come with a program, called Creative Recorder. It’s a tad clunky, but when you open it, there’s a menu that list a number of settings (Digital Audio, CD Audio, etc.). Pick the option that says “What U Hear”. Then press “Record”, and start the Midi.
I do this semi-routinely… try www.apple.com/quicktime.
I have the “professional” version, but I’m fairly certain that the free version works. Actually, I also use the Mac version, but I’m fairly certain the PC version works.
I’m sure there’s tons of software to do it, but winamp (www.winamp.com) will do it with the right settings. There’s an output option to output everything you play to a .wav file instead of to the speakers.