Thanks for the quick reply but not sure what in that thread relates to my problem so here are a few more details then you can let me know how much more information you need
The screen shots I am trying to capture are from MS Flight SimX acceleration running full screen not windowed. Captures from windowed mode work fine.
I am not running any browser while FSX or screenshot captor is running.
I am using Avast anti-virus and with it turned off I still get just a black capture.
Screenshot Captor is set to save the grabs automatically, no 'after capture' pop-up enabled. However with that pop-up enabled it shows the captured image just as a black image and when I discard the image I am returned to a black screen, not an image of FSX.
To get FSX to display I have to switch it back to windowed mode.
Hope that gives you more clues.