You can use VLC to play many more video formats not supported by quicktime.
You can also use MPEGStreamclip, and if neither it nor VLC player can open it, then I would think that the file itself is most likely corrupt.
QuickTime doesn't support a lot of video formats. There are plug-ins for QuickTime such as Flip4Mac and Perian that will extend its abilities, but even then it still won't play some formats and Perian is not even being continued by its developers beyond the next release.
A far more effective solution is to use the open-source VLC player, which not only doesn't need you to go through any complicated procedures of installing plug-ins, but natively will play just about anything you throw at it. It'll even play files that suffer from minor corruptions.