All right, I'll go through this quickly, and if you need more details, just let me know.

Step 1:

Download and install WinAmp2 and VirtualDub, if you don't already have them. WinAmp3 might work, also, but I haven't given it a try.

Step 2:

Open your .mid file in WinAmp.

From the Preferences panel (CTRL-P), select Input as "Winamp2 MIDI". Configure the plugin to use Microsoft's synthesizer as your device, and enable "Play through Winamp's output" and "Write .wav files" under output. Specify the folder where you want the .wav file sent.

Step 3:

Play the .mid file. If you have Repeat enabled, you'll have to stop it at some point, or else the .wav file will just keep growing until it devours your entire hard drive.

Step 4:

In VirtualDub, open your video file. There's no need to recompress the video if you're only adding a soundtrack, so set VDub to direct stream copy, and it'll recycle the existing video track.

Video menu --> Direct Stream Copy

Step 5:

Add your new .wav file.

Audio menu --> WAV Audio

Step 6:

Save your video.

File menu --> Save as AVI (or hotkey F7)

Depending upon the length of your animation, you might also want to use audio compression, but I'll save that lesson for another day.