QT 7.6 fails to open when movie icon clicked

Question

I just downloaded and installed the new update to Leopard (10.5.7) and now, something seems to have gone wrong with Quicktime's - to be exact QT Pro 7.6 - ability to recognize files. Until this morning, I could open a movie file (say, .mov, .MPEG, etc.) or audio file (e.g. MP3) with a control-click on the icon and choosing QT from the various players offered for opening the file. Or, as usual, I could simply double-click on a QT icon. Either way, the file would open in QT and be ready to play.
Now, suddenly, this no longer works. If I double-click on a QT icon or try to Open it with the control-click move, I see QT appear for an instant - its name appears in the menu bar at top of desktop - but it disappears just as quickly and the file is not presented in a player window.
I can still open Quicktime as a program first and then use its "Open File ...." menu command to pick a file and play it, no problem. And once the program is open, I can do the double-click move and QT files will open normally for playing.
I have re-downloaded and re-installed QT, but no luck. I am repairing disk permissions and will see if that helps. (No luck; nothing has changed.)
If anyone could enlighten me as to how OS X or its apps recognize file types, I'd be grateful. I know that in the Info pane, one can assign an app to a specific file or to all files of a certain type, but I don't quite understand where this linkage could get broken - if that is indeed what has happened on my machine.
I don't see any other problems of this sort with other kinds of files or apps.
Thanks in advance.
John

Answer 1

I had not repaired permissions, but I will try that. And in response to a suggestion from someone on the Leopard/Finder & Files discussion, I installed the 10.5.7 update and my experience is explained there, like so:
First, thanks, Baltwo, much for the response and suggested fix. I did set up another admin account and logged-in there, the problem did not seem to exist there, so I went back and installed the 10.5.7 Combo Update, as suggested, and ... well, things have become slightly more mysterious.
For instance, I have a couple of .MOV files I'd downloaded from the Web. I double-clicked on one of them and I got the same problematic behavior: Quicktime flashed open momentarily, then disappeared, and the .MOV file is left unopened. I tried the other movie file, and same thing. Then, I picked an MP3 file and control-clicked it and chose from the pop-up menu to Open it with Quicktime. Voila, it opened. And now, with that MP3 still open, I went back to the .MOV files and they both opened in Quicktime with a standard double-click.
I now closed QT and tried to double-click one of the .MOV files. Sure enough, it opened just fine. And the second one, too. I closed QT and tried this again, to confirm. But no, now these files would not open the right way.
Puzzle No. 1: In my Finder's list of Recent (App) Items, the name Quicktime shows up twice. This is downright strange, and makes me wonder if the OS is confused or seeing some set of mismatched QT components from different releases of the program.
I can erase the Recent Items list and then, even if I my trying to open a QT file fails, the name Quicktime shows up in the list. If I then do get another QT file to open, there will be a second listing of QT in the Recent Items list - as if two distinct programs had been called upon.
Puzzle No. 2: When I did open that MP3, it seemed to take considerably longer than usual, as if the computer were having to sort something out before it opened the file. And when MP3 data is loading into QT, it takes much longer than it used to - maybe 5 times slower or more. This new sluggishness in Quicktime actually has been evident to me ever since I first installed Leopard a few weeks ago. I got to live with it, but it was certainly annoying at first - and still is, if I think about it, because it makes me suspect that QT, or more likely, supporting code within Leopard, has been diddled in some way that's highly optimized for the Intel processor and in so doing treats Power PC as a lesser chip, or even an afterthought. My further trouble with opening QT files seems to point to the same thing. Certainly, I hope I'm wrong.
Puzzle No. 3: I tried to open some .DV movie files that I'd made with iMovie awhile back. Their icon shows the QT logo and they've always opened by default with QT. A double-click wouldn't open them, no matter if the QT was already open or not. With QT already open to an MP3 file, one of these .DVs opened with the Control-click-Open move, but not the others. Otherwise, nothing doing.
Maybe some of this evidence points to another possible fix, or offers insight into changes we're not being told about or that were unanticipated. I hope Apple is listening and will offer some advice or a fix.
Thanks all, John