Sony DVD RW DRU-820A has decoder error

New computer had privately built. Windows XP Pro - cannot play DVD-I keep receiving error "Windows Media Player cannot play the DVD because a compatable DVD decoder is not installed on your computer"

I downloaded Windows Media Player 11-now it tells me I need a decoder or something to convert the file to MPEG-2 or something.

Can anyone help me with this? I just want to be able to play movies and play educational DVD's with my grandson.

There is an excellent free player, from VideoLAN, available from http://www.videolan.org/vlc/

You need a dvd playing software installed to the computer before windows media player will use it
Videolan is a good one