From where to get Free software DVD decoder?

Mate, here your answer, Don't panic. Developing and certifying a DVD decoder requires that additional licensing and royalty fees be paid to the various patent holders of DVD technology. This is why DVD decoders are not available as "free" software. Usually when you buy a DVD player for a computer it comes with DVD decoder software, but what if you bought a second-hand computer or purchased a refurbished computer and it didn't come with decoder software. If you are wanting to use Windows Media player to play DVDs, I recommend purchasing Corel's DVDXPack. Its a wonderful package that instantly adds DVD playback to Windows Media Player in Windows XP. Installs the same theater-quality video and audio decoders proven by over 45 million users of WinDVD - the world's leading software DVD player.

If you are looking for a complete DVD solution, I would recommend purchasing WinDVD

Although there isn't a standalone DVD decoder that is free, there is a free solution. VLC is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, mp3, ogg, ...) as well as DVDs, VCDs, and various streaming protocols.