Oops! when You press F4 while playing a demo in ET,
A million screenshots are made in screenshots folder of mod(Not recommended, There's no sound).
A movie is single frames played one after another, Playback speed is set by the frames per second.
I use Dxtory program to record screen and sound while demo is playing(one GB movie file is produced per Minute).
Adjust video card 3D properties before recording to most brightness or movie will be dark.
Be sure to set dxtory sound volume to highest.
Then I load into Windows Movie Maker set to make the largest file in WMV.
Tory video codec lets Movie Maker preview the movie but It's not necessary.