When you create video with SolidWorks Composer the intuitive way to export it is to use the video workshop and export an AVI file. This method is fast but you lose the alpha channel. If you use the default background you will see lines instead of a smooth transition of the colours.
The solution is to use the High Resolution export and choose multiple with the video export option. This is what I use. The result is a better looking video, but it involves a few more steps. When choosing the frames per second rate (I use 30 frames per second) do not worry if you will end up with tones of Jpegs. Because SolidWorks Composer runs on Windows you probably have the Movie Maker program already installed (it comes free with Wndows 7). Upload your files to it and then export you video in one of several different file types.
Watch this video for a demonstration of this procedure: