You can make AVIs of Halflife or the mods, by recording a demo.
When the demo is record youll have to type in some sort of command, and then it will make the number of screenshots you told it to do, with the command.
ex: startmovie demoname.dem 30
30 stands for the number of screens to be made per second (FPS)
When you've done this, you should import them in Videomach or another program like it.
With that, you can Import the BMPs and make an AVI movie, and add some music or something. MAke sure you've got the correct DivX Codec, and you're finished.