[Osiris]

Nothing is more frustrating than to witness that the movie that you downloaded for hours won’t play on your computer. Maybe there is only black screen, no audio, blurred pictures or out of sync audio and video. The most likely cause for this behavior is that a audio or video codec is missing on your system.
One way to solve this would be to install one of the huge codec packages that probably install the codec that you need but will definitely install many other codecs that you will never need or even cause problems on your system.
Codec Installer is a freeware much like G-Spot wich analyzed a video file and displays audio and video codec information about it. A nice feature is the option to download any identified missing codec right from Codec Installer saving you the hassle to search for the right and current version of the codec on the Internet.

Codec Installer requires the .net framework 2.0 and runs on all Microsoft operating systems starting with Windows 98 including Windows Vista support.

source: One Tip a Day