I first installed a WMM from the Essentials 2012 pack. I added a TV show episode, trimmed it to about a minute's length, played it back. Everything was fine. Next I trimmed and added two or three more clips from other episodes. Then I added a sound track and played the entire thing again. After about ten to twelve seconds of both audio and video running, the entire computer screen went black for a second and then WMM reappeared but the video in the preview screen was now black. Audio still played. I tried playing it from the beginning again, no video. 

Next I uninstalled WMM 2012 and installed WLMM from the 2011 Essentials pack. This too had the same problem. The preview screen goes black after a few seconds of playback during the editing. 

I have Windows 7 Ultimate on my laptop. 

Can someone please tell me how to fix this error?