i'm a cinema student and i often take caps for papers (i also keep a large collection of them for my screen saver slide show.) ... i do this all the time with a program called Capture Me, which i found free on versiontracker.
the way it works best is if you have your movie playing, pause it, hit esc, get capture me open/resized to the area you want to cap, and then double click on the video window (make sure that at least an edge of it is out side of the capture me window when not full screen) to full screen, then click inside the CM window to grab your snapshot.
i was just doing this with Inland Empire when i took a break to read the forum.