If it's only for occasional use you can turn off Hardware Acceleration for your graphics under the Display Properties, Settings, Advanced, Troubleshoot menu (there's a slider), but turn it back on after. Or you can turn off "Video Overlay" under your graphics-card specific properties menu if you have one.
If it's for using a lot then get a program for it like UnsightlyEvil suggested because the other way is pretty fiddly to do again and again.
Just a note to everyone, if you sub a screen with a movie playing that has obviously been pirated (i.e. the new Matrix flick) it's going to be deleted, so don't bother. We won't be impressed and we won't think you're cool.
(although i have been wondering how to do it, i.e. cutting out a scene of something cool and showing it to a friend...never figured out how to do it though) ------ wewt.
