Just install VLC Media Player and all the codecs for audio/video files will be installed... including DirectShow decoder..... It's FREE....
VLC Media Player : http://www.videolan.org/vlc/
VideoLAN is a cross-platform media player that supports the playing of many video and movie formats, including DVD disks (both encrypted and unencrypted DVD disks), VCD, SVCD, Audio CD(no DTS-CD), MPEG, AVI, ASF, WMV, WMA, MP4, MOV, 3GP, Ogg, OGM, Annodex, Matroska (MKV), WAV, raw audio (DTS, AAC, AC3, A52), raw DV, FLAC, FLV (Flash), DivX, MPEG-4, XViD, 3ivX, H.264, H.263, H.263i, Sorenson 1/3 (Quicktime), MJPEG, Indeo Video (IV32, IV41, IV51), DVB (Satellite,Digital TV, Cable TV), MPEG encoder 4, Video acquisition., etc. It can also handle streaming video both as a client and as a server. Support systems include Windows, Linux, Mac OS X, BeOS, WinCE / PocketPC, NetBSD, OpenBSD, FreeBSD, Solaris, and QNX. The program is
open source and is released under the GNU General Public License.