Just found your post! I worked out how to do this recently! Turns out it is super easy with the DVD program I use to watch DVD's on my computer.

I use PowerDVD. Pressing "C" will do a screen capture. Depending on how your settings are set, it will either save the picture to somewhere that has been specified, or it will place the picture on your clipboard and you can "paste" it somewhere (I paste them straight into photoshop so I can play with them). To change your settings you go to Configuration (Ctrl + C), click on "advanced" and go to the "snapshot" tab at the top. Then you can choose what you want the program to do when you press "C" and take a screen capture during your DVD