Ah, DVD screenshots! :)  I use the program 
Hypersnap DX for my screen capture needs.  This is
on a PC, not Mac.  I don't know of a comparable
program for Mac. :(  Hypersnap is available at
www.hyperionics.com and it's really great.  The
latest version will take caps at 700 something by
480 and you size it down to 640 by 480 for perfect
proportioning.  

It sounds like your PC does software decoding like
mine.  Hardware decoder DVD players will let you
slow step--my roomie has one and I'm jealous of it.
:)  With software decoding you should be able to
pause anywhere you want, but it's going to take the
machine a few moments to process your pause
request, so you've got to learn to anticipate the
gap between hitting the button and actually
pausing. :)  I know, very frustrating!  

Hypersnap is nice because it just uses the Scroll
Lock button to take the shot and it has an 
auto save feature, so you may not need to pause at
all, just keep firing away with the button! :)

I have not found a DVD software player that will
do slow mo--I think it's totally dependent on
if you're doing software or hardware decoding.  
The Hypersnap program is around $50 and well
worth it.  I hope this info helps and let me know
if you want examples of shots or if you have any
questions! :)

Jen