Startup Videos
You can change the startup video for a game by placing a valid AVI file called "valve.avi" in the media folder (i.e. ../half-life 2/hl2, ../counter-strike source/cstrike, etc.).
Note:Source 2007 uses "valve.bik" instead of "valve.avi".
Tip:When converting a video file to a ".bik" file, many modern formats won't convert for a multitude of reasons. One format that will convert is ".wmv" (Windows Media File).If you are making a mod for 
Half-Life 2 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 ...
media/vid1.avi media/vid2.bik
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.
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 file size is too large, this has been tested with an AVI file over 100 MB.
You can get the BIK video encoder for free from RAD on their site at RAD Game Tools
Note:RAD Tools is a commercial product and can't be used legally without purchasing a licence for its use in your Source mods. This was pointed out to be by Jeff of RAD Game Tools.When encoding the startup video note that 
Source 2007 does not 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 support is 1024x768, as this is the size used by Valve for their startup logo and it seems to work perfectly.
To Remove The Stock Startup Video
Simply create StartupVids.txt in /media/ and leave it blank.
If you want to suppress the startup videos, you can add the -novideo parameter to your Command Line.