Is it specifically a QT file (i.e. .mov) or another type of movie file? If it's another type of file (i.e. AVI) you'll need to make sure your codecs are up to date (divx, 3ivx). Either that, or just use VLC - it's better than QT, as it plays any format under the sun.
This question gets asked and answered a lot. It is most likely that the QuickTime player can handle the file after you install the proper codec. Search this forum for "quicktime+codec" for the past answers to your question.