hmmm it is possible. first halflife has to do those many many many screenshots of the demo. this is made with a comman in the console while u watch the demo.
but be aware! 30 seconds will cost 70 mb.
then take a program, dont know the name, which converts all those nifty (i luv that word) pics into one smoothy video. then u can delete these pics.
now u can watch the video.