I dont know about copying part of a movie, but I know most of the available video converter tools allow conversion to avi or other formats from DVDs, thus saved on your harddisk. As for manipulating the converted video, some tools are available, like corel video studio, or Kate's video cutter (can google for more). You can also rip the dvd using a dvd ripper or decrypter tool, and from there work with the .vob files if available, or the ifo file for the whole movie (there usually 2 ifo files present, use the second one, which is like VTS_01_0.IFO)
