I recently downloaded a video file from the net but was unable to play it inWindows mediaplayer. Wndows Mediaplayer displayed an error that some codec was missing.. While searching for that codec I found this K-Lite Mega Codec Pack which has whole lot of codecs and that to for free. It has a Classic Media Player which is lite and nothing like the bloated Windows Mediaplayer. Thanks to K-Lite Codec Pack, I was ableto watch the movie file. 
K-Lite Mega Codec Pack includes most of codecs including the ones we normally use like mpeg, DirectShow filters, Divx and Xvid as well as other necesary codecs like Quicktime and Realplayer codecs.
If you visit CodecGuide.com you will be able to download the latest copy of K-Lite Mega Codec Pack which is around 20mb.
I have now made the Classic Media Player the default video player to watch all DVD movies and other video clips.
There are better codes! I am using the Ace mega codec. I think it has all the codecs for any video types!