If you're using Windows Movie Maker, it's also possible that you tried to upload your .MSWMM file. This is not an actual video file. An .MSWMM file is a Windows Movie Maker project file. If you're satisfied with your video, choose the "Publish Movie" or "Save Movie File" option (depending on which version you have) under the "File" menu and create a video file such as .avi or .wmv.