The Womble programs spring to mind for this type of thing. Womble MPEG Video Wizard DVD (what a name!) is good for cutting sections out of dvd video or plain mpeg files. It can do this process with minimum of reencoding.
Another possibility would be the VideoReDo TV Suite. Don’t be fooled by its name, it is a good program for working with mpeg2 found in dvd video.
Both programs have free trials.