A DVD decoder is included in Windows Vista Ultimate and Home Premium, so you can watch any DVDs without needing any add-on products. The remaining Vista versions don’t have DVD decoders; however, some decoders that run on Vista are available for download, such as the Roxio CinePlayer DVD Decoder and Cyberlink PowerDVD SE for Windows Vista. Once you purchase and install a decoder, Windows Media Player (WMP) will play a DVD.
Any Vista-compatible DVD decoder should also work on 64-bit WMP. By default, the 64-bit Vista WMP links point to the 32-bit version .The path is \Program Files (x86)\Windows Media Player\wmplayer.exe. Notice the (x86) portion. This path also applies to Windows Server 2008, if you have a suitable graphics driver. I'm using a Vista DVD player to watch a movie on my server right now!