possible DVD protection?

[xenos]

Hey,

my dad got the Mary Tyler Moore DVD set for xmas and is unable to play it on his computer (we can play it on a regular DVD player, tho).

Is anyone aware of any kind of protection (or something like that), that would prevent my computer from even seeing the disk?

The computer that I'm trying it on runs XP (home), and hasn't had any other problems with other DVDs.

I've tried using DirectDVD and PowerDVD, but again, the computer itself isn't even seeing the disks.

Any thoughts/ideas?