Alright, I have tested this extensively with other screensaver programs and it is definately something with SF 4.
If you install the new DivX 6.1, but use older version DivX avi movies in your screensaver, the movie just shows as black! i.e. blank.
There is no problem if I revert to the old DivX version.
This problem also exists if you create a new movie using DivX 6.1!
What's going on?