I want to watch a movie online, but it says I need this codex. Where can I find it and how can I install it?
check these links
Which format and container is the file in and which player are you using for playback?
Perian should still work (they stopped development) but it's a QuickTime component and therefor only adds support for playback via QuickTime or iTunes.
Another option would be VLC media player and the web plugin
i guess you need this