Frame grabs from DVD's, can anybody help?

I have seen a similar topic discussed before and this time my usual good memory, has let me down.
What I am trying to do is take a good still picture from what sometimes, are quite poor quality DVD's of early Cliff and Shads shows/concerts/tv appearances. Most of these were originally recorded on various forms of tape (betamax, vhs etc), so the originals were not quite up to todays standard, and of course when transferring or copying to DVD format, a little more quality has been sacrificed.
Can anyone suggest a good program (free if possible
) that would help me? 

What I am trying to do is take a good still picture from what sometimes, are quite poor quality DVD's of early Cliff and Shads shows/concerts/tv appearances. Most of these were originally recorded on various forms of tape (betamax, vhs etc), so the originals were not quite up to todays standard, and of course when transferring or copying to DVD format, a little more quality has been sacrificed.
Can anyone suggest a good program (free if possible

