Minimize the window into the Dock, then back out again as a quick fix.
What version of QuickTime are you using? And look for copies of the QT player that may have been moved or renamed. It looks like this can happen if you update QT, but have an old renamed copy lying around. In which case not all of the QT files get updated.
While you're at it, what Mac are you using, and what OS X version?