Cloud Convert handles the conversion on their website, making it an accessible option for basic video format changes. However, for larger files or more advanced needs, I recommend using VLC media ...

The time VLC takes to convert the video will correlate with its size, so if you're converting a lengthy video file into MP4, you'll have to wait a while. Now all you have to do is wait.

A WP3, however, is a project file and doesn't contain video. Use Photo Story to convert your WP3 file to the WMV format, then use third-party software to convert the video file to an MP4.