Most commercial dvds have a security encryption called "css". Mandriva and most other commercial distros can't legally distribute the library necessary for decoding css called libdvdcss. Penguin Liberation Front(plf) was founded to make and distribute rpms in that legal grey area that Mandriva can't distribute. You can set up plf online rpm repositories as a "urpmi" source and all of their rpms will be available to you for installation. Go here:
and follow the directions for setting up the following internet urpmi sources:
Then open a console and run:
<enter root password>
# urpmi libdvdcss
You can also install libdvdcss from within the graphical software installation center in mandrake control center if you prefer. You might also want to install the "win32codecs" package as this will allow you to play .wmv, quicktime and other proprietary video format.