I've been wondering how to do this

I know how to create in-game videos on Hl2 Death Match but I've been trying to look for Downloads for Change Dem. Files to a different video file I don't know if it can work but I've been looking and Now I am asking for help. I know there is alot of people who know how to do it SO CAN I GET HELP plz..

http://www.virtualdub.org/ here just open video file then compress it (if you want to atleast) then save as avi

You need to play back the demo and export it to your hard disk in a format that you can recompile.

Here is the beginnings of what will eventually be our film making course:

http://www.hl2dm-university.com/hl2dm_105.htm

That should get you started.