Frustratingly, the default Windows Movie Maker package only lets you export video files as a WMV. Convertors to MP4 are easy enough to find but Webm format for use with HTML5 videos is less common in downloaded software.
There's a few different tools out there to convert from MP4 to Webm, and unfortunately they can be fairly inconsistent with their results (I tried several tools that added black borders to my video, or emailed me with 'unable to convert' errors). I was beginning to lose hope until...
By far the best free tool I've found is online-convert.com, the interface is pretty basic but it does exactly what it advertises. As a bonus, it even lets you cut the video down so you can export a segment if you need to.
Here's some of the benefits:
Videos can be converted into other formats and they even provide similar services for audio, documents and images, check out their home page for a full list.