Can't play DVD/VOB files with other software than PowerDVD!

[Nomax]

Hi DVD maniacs!
Well, it's a bit more complicated than stated in the subject (but I had to do it short). Note I tried with only one DVD (Star Wars Episode 1/zone 2/PAL). When I play the disc using the usual PLAY feature in DISC mode, everything goes well. But when I try the SW DVD featured player (InterActive player, needed to access the bonus website); the copyright screens play well but once in the movie, I only get really bad movie playback with a totally scrambled picture and sorts of green squares. So why do I post this here?
Well, it does exactly the same thing with DVD2AVI when I try to play a preview of the selected VOB file, so I'm stuck with this for now....
FYI, I got an Aopen DVD-1648 drive and a Geforce 2 MX as graphic card (but PowerDVD works well in both software/hardware mode)
So...
Are the VOB files protectedon SW EP1?
Do I have a conflict with proprietary codecs?
Is it something else?
Did someone else had the case?
If so, what did he/you did?

That's the most essential questions for now in my live...

I would be eternally grateful if someone can help me in this case...

Please, help!

TIA,
Nomax