You can use this method if you have a full duplex soundcard and Winamp 2.x installed in your system... anyway i dont think WINAMP 3 can do this...
In Winamp, just go to Options > Preferences and configure Winamp2 MIDI plug-in v2.63b (as demonstrated at the screenshot below...)
Make sure Use DirectSound, Send to DSP and Send to Output options are unchecked. I prefer 48 kHZ 16 bit Stereo as output format but you can manipulate it upon your hardware capabilities and/or preferences.
Restart Winamp after having completed the configuration. Your wave file will be saved to the folder specified during the configuration, as you play the midi file, with the same name.
After completing your recordings, redo the configuration and uncheck Write WAV files to: option leaving the others unchanged for future usage.
Don't forget to restart Winamp for changes to take effect.
Since MIDI to WAV conversion is a frequently discussed topic on the Forum long time ago, I hope this would help.
"Quod me nutrit me destruit..."