Often the easiest thing to is to disable the Movie plugin entirely, so that the viewer pane falls back on the Windows Media Player preview handler (if enabled in the ActiveX+Preview+Office+Web plugin's configuration) or WMP ActiveX control (if the preview handler is disabled or fails).
The main downside to disable the Movie plugin is that it is needed for some movie-related columns in the file display (although you can replace many of them with alternative columns, so it's not usually a huge downside).
You may also be able to get the Movie plugin working again by repairing the codecs on the system, as described here: HOW TO: Enable/fix playback of various media formats in Opus -- (But the advice in there may be out of date these days. It's a while since it was reviewed.)