Chances are you have a Decklink capture card if its from High End. If this is the case, make sure you have the correct Decklink driver. Also make sure like Marty said and run m165 if you are not.

One thing to make sure of is that the Decklink is turned off in the System Settings.

You can also leave your show files alone and simply delete the Catalyst item and copy a new one into it's place.

I am guessing you are using Tiger if you bought the machine as is. It could be however that you are still running Panther.

If your machine is a G5, I recommend you stay with Tiger and do not upgrade to Leopard. I also suggest you make sure you have plenty of system memory (2GB is good).

You may also wish to change the video card to an X1900XT (the version for G5) if it doesn't have it. It might also be a good idea to change the Decklink card to an Active Silicon LFG4.