I had the same problem.  I reported it as feedback.  For some reason WMP 11
in Vista 32 bit (the only one I've used), at least in the 5456 build, can
only play non-copy protected DVD's.  

One way to get DVD's to play is to install Slysoft's AnyDVD.  The latest
version is compatible with Vista, and since it removes the copy protection
WMP 11 will then play the DVD's fine.

Another way, which just avoids WMP 11, is to use PowerDVD. I've successfully
used versions 6 and 7.  They turn off Aero during use and Windows warns of
compatibility issues, but they work.  If you buy it, you've got to give the
registration exe administrator rights in its compatibility tab in its
properties.  I found it by right clicking the Cyberlink registration shortcut
and in properties clicking open folder (formally Find Target).

Or, The DVD player in Nero 6 Ultra Edition, Nero Showtime, also works.

Or for a free alternative there is VLC Media Player.  That comes with its
own DVD player codec so you're all set.

By the way, once you install something like PowerDVD you'll get its menu of
audio options in WMP 11's DVD properties tab by clicking advanced.  But
without AnyDVD installed it still won't play normal DVD's.

There are some choices for you.

I don't always get that far before the DRM problem crops up.
Funny thing is the sound is playing and sometimes there is a picture
under the error box.

Non-copy protected files play fine.