- Some MPEG-4 encoded video stream (DivX-style), which is almost always playable by xvid. Windows installable packages can be found there. Also FFDshow has become quite popular for that. [99.9% of movies need this]
- An MP3 encoded stream. WMP can deal with that, you should have no problem. [80% of movies need this]
- An AC-3 encoded stream. The AC3Filter takes care of these high-quality audio streams. Beware of the volume level!!! Usually one should give significantly more gain from the AC3 control panel entry in order to achieve good volume levels. [>15% of movies need this]
- An OGG-Vorbis audio stream. The Ogg-vorbis decoder can be found from Xiph.org here. [<2%>
- Other codecs are significantly less common (i think), like theora, or are of course supported by WMP inherently (all WMVs, MPEG, WMAs fall in this category).
- The Adobe Flash video files (FLV) can also be played by certain players, and there are also tools to save flash video streams (youtube, myspace) as FLV files.
It is usually important to support loading of subtitle files from WMP in an automated way. This is the job of VSFilter/DirectVobSub, which can be found from the guliverkli sourceforge project. This is a significant aspect, do not ignore this.
In order to obtain audio/video codec information for a file, it is usually sufficient to right-click on it and select "Properties". The last tab shows important information, like dimensions, codecs and data rates for the stream.
The good old codec packs (Nimo, KLite) become constantly more bloated and infected with adware, spyware, and other useless junk. I am going to check out the X-Codec-Pack, which seems to have important stuff and work with minimum intrusion. In addition - and in contrast with most cases - prefer installable downloads (*.exe, *.msi files), which do write useful registry information automatically.