Some versions of Windows Vista doesn't come with a built-in DVD decoder, meaning that you'll need a 3rd party tool to playback DVDs. Most software decoders will cost money, but there are some free versions which are more than adequate.
The most simple option to get a free DVD player for Windows Vista is to download the VLC Media Player:
http://www.videolan.org/vlc/
This freeware player is a lightweight download that is able to play a huge number of media files, including DVDs. Just install this and you can now play DVD's on your PC for free!
