What is this FddShow and what are codecs?
Codecs...are some kind of driver used for video playback. As you may now (or not) there are different video formats, like mpeg, wmv, avi and so on. Each needs a specific codec to play. But while the three I mentioned are quite generic formats which are automatically supported by your normal Windows Media Player, there are also more exotic formats, like mkv, bnk or ac3. And then there are much, much more.
A codec pack installs codecs of all kinds to make sure you can play every possible format. Now while this can be helpful it can also be harmful. For instance when you don't know which one is the best to use or how to configure them.
This problem here is a perfect example: As MrEntropy said, Fallout 3 and Fallout NV are both set to NOT use the ffdshow audio codecs, yet the codec is used because I have installed it and set it as standard. Hence causing problems with Fallout NV.
So it's better to avoid these codec packs unless you're sure you really need them, or are an expert in configuring them. Most, if not all video and audio formats can be played via the Windows Media Player and the free VLC player, which uses it's own codecs without installing any of them. That's all you need!

PS: http://en.wikipedia.org/wiki/Video_codec