lol people want to blame vista for everything.
even CLEARLY things like this that have obviously nothing to do with the operating system.
a sign of someone that knows nothing about computers.
i would try putting divx formatted vids onto a dvd yourself, and seeing if you can playback them from on the disk.
find out if its just anything that wont work, or actual dvd movies.
if you have NEVER had dvds play right on that comp, maybe you lack proper codecs.
try finding some.