I got this from Macfixit.com. I did the simpler Pacifist solution successfully on 4 computers. It's way better than doing a clean install. Good luck.
http://www.macfixit.com/article.php?story=20080124101508276
Thursday, January 24th
Reminder: Downgrading to QuickTime 7.3.x QuickTime 7.4 is causing so many serious problems for so many users (no video, only audio in movies, incompatibility with third-party applications, inability to playback some file formats) that many are choosing to downgrade to QuickTime 7.3.1 (as previously noted). This obviously eliminates any security fixes and other refinements brought about by the update, but generally restores normal functionality.
There are two methods for downgrading:
Archive and Install This is the method we recommended early on in our QuickTime 7.4 coverage; it's the safest, but also more tedious. Here's how to do it:
First, make a backup of your current Mac OS X installation with QuickTime 7.4 applied, problematic as it may be. If there is an issue along the way, your valuable data will be safely intact in its current form. Next, you'll need to perform an Archive and Install process to remove all of the QuickTime update's (and potentially problem-causing) components, and replace them with the components of the older release. Unfortunately, this means you will lose some system settings and some or all third-party system add-ons.
To begin the process, insert your Mac OS X CD or DVD. Restart your machine and hold down the "C" key to boot from the newly inserted disc. Follow the on-screen instructions, and after accepting the license agreement, click "Options." Select "Archive and Install," and check the "Preserve User and Network Settings" option if you'd like to do so.
After the installation process is complete, you will be left with an earlier Mac OS X system that (hopefully) does not suffer from the problems generated by QuickTime 7.4.
Next, download and install the latest version of your OS (Leopard [10.5.1], Tiger [10.4.11], or Panther [10.3.9]) from Apple's download page. Finally, download and install QuickTime 7.3 from one of the following links:
QuickTime 7.3.1 for Leopard [10.5.1]
QuickTime 7.3.1 for Tiger [10.4.11]
QuickTime 7.3.1 for Panther [10.3.9]
Use Pacifist This method is easier, but can also potentially cause problems with system stability. Here's the method:
If you don't have it already, download Pacifist and install it.
Download the version of QuickTime 7.3.1 for your system:
QuickTime 7.3.1 for Leopard [10.5.1]
QuickTime 7.3.1 for Tiger [10.4.11]
QuickTime 7.3.1 for Panther [10.3.9]
Drag the QuickTime 7.3.1 package onto the Pacifist icon, or choose "Open Package" from the File menu in Pacifist and select the QuickTime pacakge.
With the package open, select its name (the top entry in Pacifist), click "Install" and follow the on-screen prompts, making sure the "Use Administrator Privileges" box is checked.
