Unfortunately, one of the worst things you can do with DVD and MPEG2 files is to try to convert them on a computer. The best quality possible is to use the COMPONENT VIDEO output of a DVD player and capture them to the computer with an ANALOG CONVERTER.
The reason is that the MPEG2 files and setup DVD players are optimized for playback in an analog format, so that is the best way to maintain the quality. The whole I and P frame thingy means that digitally there aren't that many whole frames in the file, but when played back analog, they are creating the illusion of the frames, and if you analog capture that back to a PC, especially in COMPONENT VIDEO, you'll get the best possible look of the video.