I am a little confused? If you mean changing the valve logo video, I can help you with that (the one with the tune and valves logo?) thats simple to change all you need to do is create a video, save it as an AVI (which I have had trouble getting steam to recognize) ...I think it needs to have a specific compression, fool around with it (I used camtasia studio to export) And used windows movie maker to help me make the video. Movie maker should be good enough to get you the kind of intro that guy had. However, you might need to export the avi in a more advanced program, one that outputs the compression (which I forget) so that steam can see the AVI file. If it doesn't it will come up as some weird error thing when you boot hl2 or whatever...
...if this is what you meant - there you go,
if not - sorry for giving you the wrong answer