To Robbyb71: I can't disable the option 'reserve edge of screen' as I've chosen "Hide off-screen when not used", and I still can't watch full screen movie!
I have this problem with 1024x768 screen. I set Object Dock as auto hide on top of the screen.
After months I still have to close Object Dock in order to see a full screen movie with Windows Media Player Classic which is really annoying!

I formated my PC and it didn't change anything!
To Killajosh: do you use Media Player Normal or Classic version?