Using Winamp's Disk Writer plugin serves equally..
Right Click the player, in the Options submenu, select Preferences.
On the left tree, select Output from the Plugins, and on the right, select the Disk Writer output plugin. Configure it to output to a directory.
Close the Preferences, open your WMA file and press play. There should now be a copy of the WMA file translated to a WAV file in your specified directory.