8200 TO PROJECTOR PROBLEM

We have 3 8200's and all three have recently become unable to connect properly to either of our projectors (Infocus and Viewsonic). We are running XP Professional. We can no longer have video on both the Notebook and the Projector, with the F8 function key no longer switching properly. With mucking about in the display settings, we can have the computer screen show on the projector, but the cursor still on the notebook. If we do get the image and cursor on the projector, there is nothing on the Notebook screen, and all is lost if the F8 key is pressed.