Open a konsole/terminal window, and type the command
that will give you the kernel name/number.
Then open the mandriva control centre (configure my computer), into the Software manager > RPMdrake + and look down the list of software packages available, there should be one that is nearly identical to the kernel name/number you got from above instruction, except it will be called "kernel-sources-2.6.whatever" not just "kernel-2.6.whatever".
Install the kernel-sources - they are needed for the nvidia installer to make the kernel module for your system.
then go to the nvidia.com
site and get the latest version of the driver for your systems architecture and the readme text file.
If you can print off the instructions all the better (you shouldn't need all the Q and A trouble shooting stuff, just the main instructions.
Then just follow them - the only bit you might need to know extra, is how to stop the X server. I used to be able to do that by doing Ctrl+alt+delete at the same time (you might need to do that more than once - the most I had to do was about 5 times).
Then when you have the text login page, you should be able to just follow the instructions - you know when you have the driver installed as it tells you (or it tells you if theres an error/problem for some reason).
Those instructions tell you what to put in the xorg.conf file, but you will have to work out which text editor you have (probably VI or VIM) so you know how to bring up the xorg.conf file and make the edit (only a couple of lines, but they're listed in the instructions).
You'll know if you've got it right, because you should then see the nvidia splash screen when you reboot.