Here are some ideas about how to convert wmv to mp4:
VLC Media Player
VideoLAN claims that their media player can handle almost any format, and it is actually true. VLC has the codecs to many commonly used formats built in, such as MPEG 1/2/4, WMV, and FLV. Once it is installed, the "Streaming/Exporting Wizard" can be used to convert any of VLC's supported formats to any other supported format. It can also perform the same task with any of the supported audio formats. VLC's feature list is rounded off by the ability to play DVD videos, streaming audio/video and in some cases cable/satellite connections as well.
Windows Live Movie Maker
Windows Live Movie Maker is available as part of a set of programs called Windows Live Essentials that includes Messenger, Photo Gallery, Mail, Movie Maker, Writer, Family Safety, and Toolbar, plus Microsoft Office Outlook Connector, Office Live Add--in, and Microsoft Silverlight. Unfortunately there is no way to download it separately, but it does include a complete feature set for creating movies. It can accept input from many different sources and then help you create a complete movie from those sources. Since it accepts MP4 videos as a source, converting is as simple as importing the video and exporting as a WMV file.
Windows Movie Maker
Windows Movie Maker is the previous incarnation of Windows Live Movie Maker. Unlike the newer version, this one runs on Windows XP. The feature set is very complete and quite similar to the new version. It can be used to stitch together video clips and streams from various sources, adding transition effects and custom text to each portion. To convert to WMV, follow the same procedure of importing the MP4 video, and choose WMV as the export format.
MediaCoder
MediaCoder is a transcoder program used to convert video and audio files from one format into another. It can be used to optimize a video for Internet upload or compress into the same format for email transmission. It also supports more advanced features such as separating the audio track from a video file and fixing corrupted videos.