Ok lets try this:Download Pacifist.
Insert the Leopard install DVD into the drive and open Pacifist. Click the “Open Apple Install Packages” button on Pacifist. This will load the DVD.
Once the DVD has loaded, there is a list of the contents from the DVD. Navigate to: XXXXX XXXXX OS Install/Contents of iChat/Applications/ . This is where iChat.app is located and what will need to be extracted/installed to your computer. Alternatively, you may perform a search on the directory listing with ‘iChat.’
Once the iChatAV application is found, highlight/select it and extract it to the computer by clicking “Extract to” or “Install.” Be sure to install iChatAV to the /Applications/ folder on your hard drive (not your user folder).
Now Repair Permissions after the re-installation just to prevent any quirky behavior by using Disk Utility located in /Applications/Utilities/. Then you should be all set to use iChatAV 3.0 (or iChat 4.0) Be sure to update to the latest version, as noted below. (ichat from an Install disk is likely to be out of Step with the OS. In tiger and Leopard so far the updates to iChat have been in the Software Updates and you need to match them.
If problems still persist, you might find it worthwhile to reinstall the
InstantMessage.framework folder (/System/Library/Framworks) and
VideoConference.framework folder (/System/Library/PrivateFrameworks).
You can reinstall either/both of these folders by modelling the steps noted above except just reinstall the folders and not the iChat.app application. You will need Pacifist for this procedure.