Have you ever downloaded a movie where you can hear only the sound and no video or vice versa? If so, it suggests that your computer does not have the required codecs to encode and decode the file.
CodecInstaller is a freeware application for Windows Vista and XP that scans and detects Audio and Video codecs installed on your computer, analyzes files to understand which codecs they require and suggests you to download and install the missing codecs from the Internet.

CodecInstaller also contains a program called MovieLauncher that displays informations about a downloaded audio/video file (author, plot, director…) along with a screenshot and a ‘Play’ button

This program can be used to analyze avi, mpeg, wmv, wma, ogg, flac, mpc, mp3, mp4, wav and wx media files and can show technical data such as width/height and ratio, length, video codec, framerate, size, bitrate, total frames, quality factor and metadata such as title, album, year (if available)

Download CodecInstaller for Vista and XP here

Note: The software requires Microsoft .Net Framework 2.0