Did you find that when you try a normal Print Screen that you just end up with a black box that's the size of the DVD movie's window? You can capture the output, but only if you disable any sort of hardware acceleration in the software you use for playing the DVD (if there's one available). Then you can just use the image editor of your choice to convert the image. I do believe that there are tools that let you inspect the files from a DVD individually, though I wouldn't know what to recommend. Check out www.divx-digest.com
As I'd said, you need to disable hardware accel in the program you use to play your DVDs. Normally the program will try and write information directly to the buffers on your video card rather than going through Windows to display it. While it allows for smoother movies it causes the behaviour you see there. Likewise, if you can disable hardware acceleration you should be able to get a clear picture.
Well, it *used* to work in WinDVD, though now it's slightly harder to accomplish. After going to Properties->Display you have to deselect "Hardware Motion Compensation." Then, when you go to the scene you'd like to capture, press the "P" key on the keyboard. It saves it in a folder in the WinDVD folder within Program Files directory.
I can't speak for other players, however, as WinDVD's the one I use.