Wrong file types there. An avi file is a video file interwoven with sound while wma files are the format for Windows audio files. Microsoft originally intended to have the wma format in order to compete with mp3s. A full description can read at http://en.wikipedia.org/wiki/Windows_Media_Audio
The details on avi files are seen at http://en.wikipedia.org/wiki/Audio_Video_Interleave
avi is a format for video, wma is a format for audio, if you want to convert avi to wma, you need a software. I know a software named AVI MPEG WMV RM to MP3 Converter which is a powerful tool to convert all video and audio files to MP3, WAV, WMA and OGG formats.