How do I convert a WMV/WMA file to an mp3 file?

Okay so in windows movie maker I used to be able to take a video file and put it into the audio section and make it, itself be the audio for the video. But now that I have windows 7 I cannot do that. There is no audio section it seems, I can import audio, but not WMV/WMA for audio. Is there any way I can convert the WMV/WMA file to mp3 without having to download a bunch of junk?