DVD MPEG NTSC compliant movies are always in 16:9 (widescreen) aspect ratio.
Refer to the URL:
http://www.deskshare.com/vem-help/ConfiguringMPEGProperties.aspx for more information on the available MPEG standards.
To maintain the aspect ratio of the original clip:
1. Create a new project in 720 x 480 preview dimension.
2. Add the clip to the timeline.
3. Double click on the clip to set its Stretch Mode and select an appropriate mode.
Now, Make Movie will maintain the original file's aspect ratio