Download K-Lite and throw it on a network share that is accessible from the computer accounts you want to deploy it to. Then create a startup script that runs this
\\server\share\klcodec.exe /verysilent /norestart
If you need to customize it more, simply run klcodec.exe -makeunattended to generate an unattended answer file
Link to k-lite:
http://www.codecguide.com/download_kl.htm
Edit:
Might want to add something to the prvious script. It wouldn't be a bad idea to check if it's already installed before installing it. This won't be perfect, but it should work on 99% of your computers:
IF NOT EXIST "C:\Program Files\K-Lite Codec Pack" \\server\share\klcodec.exe /verysilent /norestart
If you have 64 bit machines in your environment (which I don't have, so I'm winging this a bit), then it might look like this:
IF NOT EXIST "C:\Program Files\K-Lite Codec Pack" IF NOT EXIST "C:\Program Files (x86)\K-Lite Codec Pack" \\server\share\klcodec.exe /verysilent /norestart
Not pretty...I know...but it works most of the time