I believe it is happening due to the Power DVD software.
When you select the desktop the video display minimises so you can view the desktop clearly. As you are using the second monitor it sees the desktop of both monitors as one.
You could try EasyWorship. It is design for something similar to what you are trying to do:
http://www.easyworship.com/
You can also try MatrixDVD. It has dual monitor support, but it isn't the best quality video:
http://www.xj-spark.com/