This is General Hardware so this might be better suited for the software section, however, this is a simple thing to do.
Get Winamp. I use v2.81. In Winamp, hit Ctrl+P, or get to Preferences with the menus. Under Plugins, go to Output. Select Nullsoft Diskwriter, and hit configure. Tell it where you want the WAV files to go; you can probably leave the rest of the options alone. Close out of that; select the files you want to convert, make sure that auto-rewind is off, and hit Play. You won't hear the file playing, but when it does play through completely (it should do it in maybe 10s, possibly longer), the WAV file will be in the directory you chose.