From Media Player Codec Pack:
The Media Player Codec Pack is a free simple to install package of codecs/filters/splitters used for playing back movie and music files. After installation you will be able to play 99.9% of files through your media player. Additional file associations are applied to make Windows Media Player play BDMV (unencrypted BluRay/AVCHD), EVO (unencrypted HD-DVD), NOTE: BluRay and HD-DVD playback is very basic and requires both RedFox AnyDVD HD and compatible hardware. MKV, M2TS, MP4, VOB, OGM, WEBM, FLV, and NUV movie files, as well as AC3, DTS, FLAC, APE, OGG, WV, TTA, OFR, SHN music files, and finally MO3, IT, XM, S3M, MTM, MOD, and UMX tracker audio files. Compression types supported include AVC, H.264, x264, H.263, H.261, DivX, XviD, Flash Video, VP3-8, Morgan JPEG, MJPEG, MPEG4, MPEG2.
The package detects the best proof tested settings with additional options for CPU, GPU, and DXVA compatibility. Audio options during install allow users to set output from basic 2.0 channel stereo, right up to 7.1 channel output. Digital audio S/PDIF / HDMI passthrough for home theatre systems can be set by the user to output Dolby Digital AC3, Dolby Digital Plus E-AC3, Dolby TrueHD, DTS, DTS-HD, even multichannel non-AC3/DTS audio (for example 5.1 channel FLAC) can be set to encode as AC3 and output to digital S/PDIF / HDMI. Included in the package are a number of encoder codec's, allowing you to encode files in formats such as DivX and XviD in your favourite encoding application. Please note that protected formats may require a specific player. 32-bit and 64-bit version of Windows Media Player are supported as well as most other players. The package supports 32-bit and 64-bit versions of Windows.