Quicktime files blank

Did you tell the Finder to open them with something else? Or maybe let Real Player take them over or something?

Try opening up a Get Info window on one, and see what it says under "Open With". If it's something other than "QuickTime Player", then set it to that, and click "Change All", which should theoretically fix it.

If you haven't restarted recently, you might try that too--10.3 sometimes had problems with screwed-up icons in the Finder.