I use the following method all the time and it works well.
How to convert a stand alone burned disk to a PC authored disk...
Using Smartripper 2.41 (although the instructions are a little different - you can do the exact same thing with DVD Decrypter):
1. Throw the dvd that you want to demux into the drive and open SmartRipper 2.41.
2. You'll see a window open up as the program analyzes the dvd - bunch of text will go by (vob listings, etc.).
3. Change "target" at the bottom of the GUI to your target folder (best if it's an empty folder).
4. Rip method is "movie" (top left) which is the default.
5. Check "enable stream processing," (just above the green window) and pick which elementry streams you want to "demux." All three will be checked by default.
6. Check on the video NTSC, and then to the right of that under "ID=0xE0," check "Demux to extra file."
7. Check the .AC3 line, and then to the right of that under "ID=0xE0," check "Demux to extra file."
8. Click "start."
After SmartRipper is done ripping, go to the destination folder and look for the two elementry streams (.m2v and .ac3 files). There will be a bunch of other files in there, however, you will only need the .m2v and .ac3 files. They are easy to pick out because they are the two largest files (and have the longest names). Delete all the other files, and for sake of ease, rename the .m2v and .ac3 files with the same name (but don't change the extension!).
If your authoring program doesn't accept elementary streams, you can mux the .m2v and .ac3 files into one mpeg2 file using TMPGenc - under MPEG Tools. Open TMPGenc, and close the opening window (project wizard), and then click file --> MPEG tools. It should be defaulted to "simple multiplex." Under "type" select "mpeg-2 program (VBR)." Select your video and audio input files (the .m2v and .ac3 files) and select and output file. Click on run, and in about 15-20 minutes, you should have an mpeg2 file that is ready to rock. Or better yet, get this amazing little program called Womble MPEG editor. It's different from most other mpeg editors in that it allows you to manipulate the mpeg files much more easily than most other programs. Using it is pretty self explanatory, so I won't go into detail.
You can put that remuxed mpeg2 in DVD Architect (or any other authoring program) and away you go. You can take the finished mpeg2 and drop it back in Womble to create some short clips to use for motion menus. Or you can simply use still pictures for your menus.
Good Luck!