System: Windows 7 Home Premium x64, brand new HW.
The machine rejects to play a few DVD-s. It is consistent on what to reject. Some DVD-s are fine, they are always fine. Some are rejected, they are always rejected.
All DVD-s played fine on my old XP machine.
The DVD-s are also backed up as ISO files on the hard drive, and can be played via virtual DVD drives. The result is the same. The non-tasty DVD-s are rejected, the tasty ones are accepted to be played via the virtual drive.
I have installed no extra codecs or player software besides Winamp and iTunes for movies. So I just use the default Windows Media Player and the codecs shipped with Windows 7.
Some more hints: The computer has been purchased in Europe, as well as the DVD-s, so I do not believe that this is a region issue. One of the DVD says in it title NTSC, but I have no idea how could digital DVD content be NTSC (or PAL).