DVDs won't play in Windows Media Player 9

[Graham]

Does anyone know why when I try to play a DVD in WMP9 I
get a gabled screen and an error message "cannot play
this DVD because a problem occurred with digital
copyright protection"? My player is set correctly for my
region and there are video codecs installed.
I'd be grateful for any ideas!

[Nero]

http://support.microsoft.com/?kbid=283620