John
Start a new project, drag a colour board to the timeline and create your opening credits, etc. Make sure there is nothing else on your timeline.
Go to Produce, Step 1 - Create a File, Step 2 - .AVI , Step 3 - Click on the ellipsis ( ... ) button (top right) and choose a name and location for the file you are about to produce. Click on Start Rendering and wait for the Congratulations message. Remember where you saved this file. Close this project.
Now go back to the project you have already spent a lot of time on and simply delete whatever you have done with your opening credits, but make sure you make a backup file first. When this is edited to your satisfaction, go to Create Disc. Make your menu as usual (Chapter 1 should be your real chapter 1 now) then go to the First Play Video section at the bottom of the preview screen and click on the first icon ("Add first play video") and browse to find your opening credits file you created earlier. Click on it and it becomes your First Play Video - it should play as soon as you put the DVD into your machine, before your menu appears.
Remember to back up your file before you start tinkering with it. Generally with these projects it is wise to back up frequently - it can save a lot a agony in the case of a problem later.
Hope that helps
Robert