Can't play movie DVDs in Natty on HP laptop

[MR ZenWiz]

I've written recently about some interesting situations with a recent
acquisition of mine (a used HP NW9440 laptop) and all the fun I've had with
it so far (ick).

It turns out that not only does it not play Avatar, it won't play any normal
DVD with a movie in widescreen on it.  It plays 4x3 ratio DVDs without any
trouble, and I can play the .avi rips I made on my desktop with the laptop,
but not the DVDs.

I'm using VLC and mplayer, both of which fail.

These DVDs play just fine under Windows 7 on the same laptop, so I'm
reasonably sure it isn't the hardware, but under Natty, no such luck.  The
DVDs also play perfectly on my desktop, but that's never been a problem.

My VLC and mplayer installations are straight from synaptic, so these are
standard installations AFAIK.

What am I missing?
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