Now that I've downloaded the older version of Quick Time, how do I tell my computer to use that version to open my file?

Drag the file to the QuickTime 7 application, or control-click the file, choose Open With, and change the setting.

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To do it just once right click on the file --> open with

To do it for all of that kind of file:

Mac OS X: Double-Clicking a File Opens the Wrong Application - http://support.apple.com/kb/TS2291

-= Changing the application used to open all files of a certain kind (written with reference to OSX 10.4 but may not have changed) =-

1) In a Finder window highlight a file of the kind you want to change the application to open that kind of file.

2) While that file is highlighted, select File > Get Info or press command (apple or propeller icon) + i to get a file information window.

3) In the lower part of the info window there is an "open with" menu with a list of applications.

4) If your application is already in the box then it is the default application for opening that kind of file and you don't need to do anything more. Close the get info window.

5) If the application showing in the menu is not the one with which you wish to open the file then select a new application. If your application does not appear there then select the "other..." and track down the application (usually in the Applications folder at the main level of the computer).

6) If you wish to change all files of this type to open with this application in future, make sure the "change all" button is selected.

7) Close the get info window.

There should be two copies of the Quicktime application in your Applications folder.

As I recall the 'new' version of QuickTime Player is in the /Applications folder but the old on ends up in /Applications/Utilities folder.