Phil
I'm not expert in this field, but remember encountering a similar problem a long while back. I found a definition similar to this:
Quote:
WMV
Windows Media Video
Microsoft's file format and codec for video. It is designed to work primarily with Microsoft Windows Media Player software on Windows PCs and Windows Mobile smartphones and Pocket PCs, although some other media player software also supports this format.
There are different versions of WMV, and not all media player software supports the newest versions.
(The emphasis is mine.)
I changed format (I think to AVI) and had no problems. Your project might not allow you the same latitude.