Pre-installation instructions:

It is highly recommended to first uninstall other codec related packages before installing this package. This way your system stays clean and potential problems will be avoided. This installer is able to automatically detect a lot of codec packs and it will ask you if you want to uninstall them. You can remove additional ones by going to: Start Menu > Settings > Control Panel > Add/Remove Programs (a.k.a. Software).

Installation instructions:

Below on this page you can find some information about all components included in this codec pack. Please read this information thoroughly. If you are unsure what to install, then install with default settings. Do NOT simply install everything.

Unattended installation:

Run the setup with the command-line parameter -MakeUnattended to create an unattended setup. A wizard called "Unattended Install Maker" will appear. Follow the instructions given in the wizard.

Features:

Media Player Classic [version 6.4.8.4]
This is a full-featured player. Has internal decoding support for DVD, MPEG-2, MP3, AC3 and DTS. Has internal support for opening Matroska and Ogg containers. It also plays QuickTime and RealMedia (if those codecs are installed).

DivX Pro [version 5.2.1] [Decoding]
This is required for playing DivX movies.
DivX Pro [version 5.2.1] [Encoding]
This is required for encoding movies in the DivX format.
XviD [version 1.0.3 build 2004-12-20] [Decoding]
This is required for playing XviD movies. XviD is currently the most popular format.
XviD [version 1.0.3 build 2004-12-20] [Encoding]
This is required for encoding movies in the XviD format.
3ivX Pro [version D4 4.5.1] [Decoding]
This is required for playing 3ivX movies. Also supports various other MPEG-4 variants.
3ivX Pro [version D4 4.5.1] [Encoding]
This is required for encoding movies in the 3ivX format.
Windows Media 9 VCM [version 9.0.1.369]
This is required for playing movies and audio that are in the Windows Media 9 format.
On2 VP3 [version 3.2.6.1 DT]
This is required for playing VP3 movies. These are not very common.
This is required for encoding movies in the VP3 format.
On2 VP6 [version 6.4.2.0] [Decoding]
This is required for playing VP6 movies. These are not very common.
On2 VP6 [version 6.4.2.0] [Encoding]
This is required for encoding movies in the VP6 format.
On2 VP7 [version 7.0.8.0] [Decoding]
This is required for playing VP7 movies. These are not very common.
On2 VP7 [version 7.0.8.0] [Encoding]
This is required for encoding movies in the VP7 format.
Ligos Indeo XP [version 5.2820.15.58]
Required for playing movies that have the Indeo 5 format.
Intel Indeo [version 4.51.16.2]
Required for playing movies that have the Indeo 4 format.
Intel Indeo [version 3.24.15.03]
Required for playing movies that have the Indeo 3 format.
Intel I.263 [version 2.55.1.16]
Required for playing movies that have the I.263 format.
huffyuv [version 2.1.1 CCE Patch 0.2.5]
Required for playing movies that have the huffyuv format.
Required for encoding movies in the huffyuv format.
huffyuv is a lossless (no quality loss) video compression format.
MS MPEG-4 [version 4.1.0.3927]
Required for playing (and encoding) movies that have the MS MPEG-4 format.
DivX ;) MPEG-4 Low and Fast motion [version 4.1.0.3927]
This is an old version (3.11) of DivX. Only required for those who want to use it for encoding something. 
Cyberlink DVD decoder [version 6.0.0.1417]
This can be used to play DVDs with Windows Media Player, but only in combination with Windows XP.
Not required if you use Media Player Classic, because it has an internal DVD decoder.
Elecard MPEG-2 demuxer [version 2.0.84.30429]
MainConcept MPEG-2 demuxer [version 1.0.0.54]
Ligos MPEG-2 demuxer [version 4.0.0.77]
Windows already has a MPEG-2 demuxer by default so normally you won't really need these.
The Ligos dexumer only works in combination with the Ligos decoder.
Ligos MPEG-2 decoder [version 4.0.0.77]
Cyberlink MPEG-2 decoder [version 6.0.0.1402]
MainConcept MPEG-2 decoder [version 1.0.0.56]
Required for playing MPEG-2 (and SVCD) movies. Ligos and MainConcept will also decode MPEG-2 audio streams.
Not required if you use Media Player Classic, because it has an internal MPEG-2 decoder.

Fraunhofer MP3 DirectShow decoder [version 1.9.0.311]
Required for playing movies that have a MP3 audio stream.
AC3 DirectShow decoder (AC3filter) [version 0.70b mod]
Required for playing movies that have an AC3 audio stream.
Use this version if you have problems with the less stable 1.01a version.
AC3 DirectShow decoder (AC3filter) [version 1.01a rc5]
Required for playing movies that have an AC3 or DTS audio stream. Also required for DVD audio.
AC3 ACM decoder [version 0.8]
Required for opening movies that have an AC3 audio stream in applications like VirtualDub.
Vorbis DirectShow decoder (CoreVorbis) [version 1.1]
Required for playing movies that have a Vorbis audio stream.
AAC DirectShow decoder (CoreAAC) [version 1.0b9 build 2005-02-24]
Required for playing movies that have an AAC audio stream.
Not required if you install the 3ivx AAC decoder.
AAC DirectShow decoder (3ivX Pro) [version D4 4.5.1]
Required for playing movies that have an AAC audio stream.
Not required if you install CoreAAC.
AAC DirectShow encoder (3ivX Pro) [version D4 4.5.1]
DirectShow encoder for AAC audio.
MusePack DirectShow decoder [version 1.0.0.3]
Required for playing MusePack audio files.
Monkey's Audio DirectShow decoder [version 1.00]
Required for playing Monkey's audio files (.ape).
Monkey's Audio is a lossless (no quality loss) audio compression format.
WavPack DirectShow decoder (CoreWavPack) [version 1.0.3]
Required for playing WavPack audio files.
FLAC DirectShow decoder (CoreFLAC) [version 0.4]
Required for playing FLAC audio files.
FLAC is a lossless (no quality loss) audio compression format.
Voxware MetaSound DirectShow decoder [version 1.0.0.12]
Required for playing movies that have a Voxware MetaSound audio stream.
DivX ;) Audio [version 4.2.0.0]
Some DivX 3.11 videos use this.
LAME MP3 Encoder [version 3.96.1]
This can be used to encode MP3 streams/files.

MP4 DirectShow splitter (made by Haali) [build 2005-08-02]
Required for opening audio and video streams in a MP4 container (.mp4 files).
Ogg DirectShow splitter (made by Tobias) [version 0.9.9.5] and Ogg Vorbis DLLs [version 1.1.1]
Required for opening audio and video streams in an Ogg container (.ogm files).
Not required if you use Media Player Classic to play these types of files.
Ogg DirectShow splitter (made by Gabest) [version 1.0]
Required for opening audio and video streams in an Ogg container (.ogm and .ogg files).
Not required if you use Media Player Classic to play these types of files.
Matroska DirectShow splitter (made by Gabest) [version 1.0.2.6]
Required for opening audio and video streams in a Matroska container (.mkv and .mka files).
Not required if you use Media Player Classic to play these types of files.
Matroska DirectShow splitter (made by Haali) [build 2005-08-02]
Required for opening audio and video streams in a Matroska container (.mkv and .mka files).
Not required if you use Media Player Classic to play these types of files.
Matroska DirectShow muxer [version 1.0.0.9]
Can be used to mux audio and video streams into a Matroska container.
DirectVobSub (vsfilter) [version 2.33 / 2.36]
DirectVobSub is a tool for displaying subtitles. Your subtitle file(s) should have the same filename as your video file (for example: MovieName.avi - MovieName.idx - MovieName.sub) and should be in the same folder as your video file. Then the subtitles will automatically be displayed in most players. Use version 2.33 if you have problems with the less stable 2.36 version.
SHOUTcast Source [version 1.0.0.1]
This can be used to play SHOUTcast streams in any DirectShow enabled player.
Matrix Mixer [version 0.30b]
Matrix Mixer can be used to amplify the sound of a video file. It also can mix stereo sound to multiple channels (useful for 4.1 and higher speaker sets).
It is NOT recommended to install this filter, unless you really need it.
Morgan Multimedia Stream Switcher [version 0.9.9]
Morgan Multimedia Stream Switcher can be used to switch between audio streams in a video file that has multiple audio streams. These types of video files are not very common. Both Media Player Classic and BSplayer have this functionality built-in. This means that you do NOT need this filter if you use one of those players.
It is NOT recommended to install this filter, unless you really need it.

ffdshow [cvs build 2005-08-03 by CelticDruid]
This is a collection of filters for decoding various audio and video formats. For example: DivX, XviD, MPEG-4, H.263, H.264, MPEG1, MPEG2, MJPEG, CinePack, TrueMotion, DV, ASV1/2, VCR1, VP3, huffyuv, Theora, AAC, AC3, DTS, MP2, MP3 and TrueAudio.
You need this for decoding the new H.264 codec (MPEG-4 avc). It is also required for playing MPEG-4 video that is inside an .mp4 file.

GSpot Codec Information Appliance [version 2.52 beta 1 build 2004-09-15]
GSpot is a tool that can tell you exactly which codecs are needed to play a certain video file. This is very helpful when you have troubles playing a video file.
Sherlock [version 2.0]
With this tool you can generate a list of all installed codecs. This list can be saved as a text-file.
VobSubStrip [version 0.11]
With this tool you can strip languages from VobSub subtitle files (.idx and .sub files).
GraphEdit [build 041201]
GraphEdit is a visual tool for building and testing DirectShow filter graphs.
AVI Fixed [version 2.0b1]
This tool can rebuild the index section of .avi files. This can make files with broken frames playable. In some cases it will also solve synchronisation problems and incorrectly displayed movie lengths.
FourCC Changer
The FourCC (Four Character Code) Changer is a program for changing the FourCC of a video file, which is used to identify which codec should be used to decode the file. This tool is for expert users only.
Bitrate Calculator
This can be used to calculate the bitrate to use when encoding a movie.