Guess, you are new on this..
1. d/l and install winamp
2. go to "option"-> "preference"
3. at "Plug-ins", select "output"
4. on right of the window select "Nullsoft Disk Writer plug-in"
5. Then DOUBLE click "Nullsoft Disk Writer plug-in", will bring you to the directory. (select where you want your mp3 to Wave file to be stored)
6. click "ok" after you select the location.
so....go you your mp3 file and play it. (you will see winamp is in action, but no voice. DON'T worry, that is how it goes)
When you done, make sure change your "preference" back to default.
p.s. are you trying to burn into CD?
if so, simply use NERO, it will automatically convert your MP3 to WAV when buring "AUDIO CD".