You need to go to the Options at the bottom of the View tab on the MPC

When the window options open go to Playback / Output

for the Output option I have the following picked

* Directshow Video - EVR custom pres

* Real Media video - Direct X 7

* Quicktime Video - System Default

I'm no expert but these settings have worked for all the caps I have produced for the films I've posted here.

It may depend on the video type you are trying to produce previews for, if my suggestions don't work I'm sure one
of the technical whizzkids here will be able to suggest something.