First of all, hitting the print-screen key in 
windows will put a copy of what's on the screen 
into the clipboard (you can just paste it into 
whatever image editor you like).

Now, you may notice that the area where you dvd 
image should be shows up as blank or black -- 
that's most likely because your video card is 
displaying the image in overlay mode.  If that's 
the case, then you can go to system properties 
(right click on my computer), go to the 
performance tab & click on graphics, then move 
the slider for hardware acceleration from "Full" 
down to "None".  This should disable overlay mode 
& allow you to capture.  Of course, as you might 
guess, you're going to want to set that back to 
full as soon as you're done, since it'll slow 
down other things as well.

Ideally, you'll be able to find a function within 
your dvd software that will let you capture 
directly and not have to go this route, but when 
all else fails, you could try this.