Not sure if this is helpful, pwantzel, but frequently when I do automatic screenshots, they don't show up on the display unless I either re-save the entry or navigate to a different movie and then back again. I know they're there, it's just a refresh issue or something.
Also, I've occasionally had fffmpeg start up numerous copies of itself, which just mucks with everything -- in those cases, I've shut down PVD, opened up Task Manager and killed all process entries of fffmpeg. Then started again. If nothing else is working, maybe open up Task Manager and see if you've got more than one instance of fffmpeg running (under "Processes", not "Applications").
I'm on Windows XP, though, not sure how much of this (if any of it) applies to Vista. Could be that OS requires a different version of ffdshow. Then, of course, the other option is that you could just use MS DirectShow for screenshots, I suppose -- as long as you're only working with AVI files.