This is a sample video clip which Powerdirector loads each time you open it up.
Look on your hard drive and see if this file exists. there should also be 5 .jpg pictures in this directory.
If not, I would just copy any other .mpg file you may have on your computer into this direcory and name it aquarium.mpg just to keep the message from popping up.
I guess the other option is to re-load powerdirector but I would go for the quick fix.