On the right, select "Nullsoft Disk Writer plug-in"
Click on "Configure" to choose a folder where you want to store the WAV file(s)
Click on "Close" to close the Preferences
Now play your SPC's. Instead of hearing the music, the sound will be saved as a WAV file.
Instead "Nullsoft Disk Writer plug-in" you can also use other output plugins offering direct conversion to MP3 ("Nullsoft MP3 Output plug-in") etc. Just look into Winamp's plugin archive for the corresponding plugins.
After you're done, make sure you change the output back to the "Nullsoft waveOut plug-in" or "Nullsoft DirectSound plug-in".