PC Review have a new article on Windows Vista Codecs which recommends the best options available to get maximum video compatibility on your system
The K-Lite Codec pack is the most popular codec pack available at the moment and works flawlessly on Windows Vista. The main benefit of this package is that you can choose the broad type of pack you wish to install (either Basic, Standard or Full). Most users will want to install the standard package, as this should play almost any video file available. If you find a video format that is not supported, it is easy to re-install the full pack at a later date. The full pack is primarily designed for people that need to encode as well as decode (for example, in a movie making package), so is overkill for the vast majority of users.Vista Codecs
