You can first try something : reboot with your installation CD/DVD inside the cd-rom reader.
Boot the mandriva installer.
Chose "repair", let the startup thingies run, it will take a time
In the hardware description menu, chose "generic" as a screen, and make sure to precise correctly (perhaps once again) what is your graphical card.
Don't modify other options, reboot the computer after installation ends (without the installation dvd in the reader)
Tell us if that solved your problem, you just performed a complete reinstallation of your screen and graphical card drivers
