I use windows movie maker alot to create home movies. I hope I am able to help you in this matter. You stated that "Verify that the original source files used in your movie are still available, that the saving location is still available, and that there is enough free disk space available, and then try again.'"
Well the problem I see you having may be that you are removing the source files, this would be the video or pictures you imported into windows movie maker from your computer. These items need to remain in the original spot you imported them from until you are totally complete with the project. Since you are not saving the project to your computer then the movie maker will need to read the original during burning.
Secondly, I do not see an option for burning to dvd in the windows movie maker. Just save to cd. Are you using another program to burn this? You may want to try to save the movie onto your computer and then use another program like nero to burn to dvd. Can you tell me how you are trying to burn this to dvd? This would help. Thanks