Any media player with AAC playback capability can at the very least convert it to WAV from where you can then reencode into MP3. Winamp has a built-in wav writer in its output plugins. You might even be able to find a direct converter plugin.
After Mezarashi's suggestion about WinAmp I did some looking around and came across a forum for Winamp. I found a post that put me onto this application JHymn. I have yet to try it, but it looks promising. I also agree with what the author says about DRM and all.
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This might be a stupid question, but why don't you just download mp3 copies of the songs you want to convert? In fact, in the future, why not just buy from mp3.com or another site that sells mp3s? Are their prices different?