Vista should have a built in decoder for dvds. Not a good one, but it is there. Unless you have Vista Home Basic or Vista Business.
You can buy a dvd player if you like, and it will include a decoder. Or just do a test drive with one of their trials. PowerDVD, WinDVD or Arcsoft TotalMedia Theatre are three popular commercial programs.
There are other alternatives, like the Nvidia PureVideo Decoder (not free).
And you can always use a free player like VLC, Media Player Classic HomeCinema or Gom Player for playing dvds. None of these three will enable playback in WMP 11, but will play dvds by themselves.