My understanding, which may be flawed, is that there are two separate, distinct QuickTimes: QuickTime 7, which is the older QuickTime and is used to play old QuickTime movies, and QuickTime X, which is a modern, Web-standards and new-codec player that's used to handle things like H.264 files. You can have both on new versions of OS X (10.6 or later); 10.5 and before are limited to QuickTime 7 but can't use QuickTime X.