From Valve Developer CommunityYou can change the startup video for a game by placing a valid AVI file called "valve.avi" in the
../counter-strike source/cstrike, etc.).
If you are making a mod for HL2 you can place any AVI or Bink Video (.BIK) file in your mod's
media folder. In order for the video to play when you start your mod you will need to create a StartupVids.txt file in
media. Inside StartupVids.txt you simply specify the filename(s) of the video(s) you want to be played on a separate line. For example ...
Keep in mind that an AVI video will need to be encoded using the Cinepak codec. It is not known whether audio codecs other than Microsoft ADPCM are supported.
If you want to suppress the startup videos, you can add the
-novideo parameter to your Command Line.
DivX is also supported, however there appears to be a size limit to the startup video, so if a video is not played then its filesize is too large, this has been tested with an AVI file over 100 MB.
You can get the BIK video encoder for free from their site at RAD Game Tools
When encoding the startup video note the Source 2007 Engine does not appear to support all resolutions of BIK video. When using an unsupported video size (let's say 720x480) only the first few seconds of the video will display. After that it will freeze and you will only hear the video's sound. Another possibility is that the video will not play at all. One size that Source 2007 does like is 1024x768. Personally I have not tried other sizes, but this is the size used by Valve for their startup logo and it seems to work perfectly.
To Remove Valve Video
Simply create StartupVids.txt in /media/ and leave it blank.