Vista - can't play certain DVDs!

this is my first post...

originally wanted to post it in Media Center category but it seems like media player is not working properly either...

the problem i have is when i put a retail DVD and try to play it, Vista does not recognize it as DVD, instead it thinks it's a blank DVD.
it does see folders but doesn't see files in them.
to rule out by drive issue, - i can play same disk under XP. (i have dial boot)

the weird thing is that it happens only with certain DVDs not all and it's not related to 'enhanced DVDs' issue i read about. for example, i'm trying to play "Gone Baby Gone" which is not enhanced DVD.

any thought or suggestions are greatly appreciated!