A couple questions.
1. Did you download and install the Nvidia drivers from Nvidia's website? Do so following their instructions. http://www.nvidia.com/object/unix.html. Choose the architecture that matches yours.

2. Did you set the correct horizontal and vertical frequencies for your monitor? Double check these as sometimes they mysteriously change during an install.

I currently use SuSE. I haven't used Mandriva/Mandrake in about 4 years so I'm not too familiar anymore with the way they do things.
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Welcome to the world of Linux, and enjoy the ride!