How to capture a screenshot from a playing video.
Print screen doesn't work.

Several good media players can make screenshots from videos ("good" excludes WMP).
Alternatively, use a screenshot tool like HyperSnap-DX.

The reason print screen doesn't work is because you have overlay enabled, if you disable that it will work, but I wouldn't recommend you do it that way. I would go to http://www.virtualdub.org and download the latest virtualdub and open you video in there, then seek to the frame that you want to capture and hit ctrl + 1 and it will copy it to your clipboard. An alternative way of taking a screenshot is to use ffdshow to playback the video and tell it to grab a frame or any number of frames between a start and an end frame, you can find ffdshow at sourceforge.

Thanks for the sugestions.
I will try it after the weekend.