To convert m4a to mp3:
Download i-tunes click here
Go into i-tunes then Edit/Preferences - Advanced/Importing and set mp3 and bitrate, then when you add a track to the library right-clicking gives you the "convert" options.
You might want to watch out for the applications i-tunes tries to add to your start-up settings. Blocking them with your firewall is the easiest option.
I did find WMP codec/addin that let the user play m4as directly, but it was trial period only, so you'd need to convert after that anyway.