In MediaLab, try using an essay item. You can capture and record each time they hit a particular key by using the k parameter. See:

www.empirisoft.com/medialab/help/queparameters.htm#key

To display the video, you simply enter it as a background video. See:

www.empirisoft.com/medialab/help/quebackground.htm

You can also adjust the location of the essay window so that you don't see it--e.g., using(t2000) will move it off the screen completely in most cases.

Let me know if you have any trouble with it!