Were you not asked to specify the username and password when you installed Mandrake?
You need to login using your "root" username and password. ie. the username is "root" (without the quotes!) and the password is probably "root" as well.
Once you have logged in, you should have a DOS-style console in front of you. To edit the file:
1. Type in "vi /etc/modprobe.preload"
2. Use the arrow keys to move up and down to find the place where you need to insert the text 'nvidia'
3. Press I (That is, the key for the letter 'i' on your keyboard) to enter INSERT mode
4. Type in the text nvidia
5. Press Shift + : (Shift plus the Colon key) - You should get a prompt at the bottom of the screen
6. Type in "save /etc/modprobe.preload"
7. Type in "quit" to exit vi (the text editor program)
8. Press Ctrl+Alt+Delete to reboot the PC
9. Login again, and type in "modprobe nvidia"
You may need to reboot again after this step. If you still have a plain console in front of you, type in "init 5" (when logged in as root) to go into Graphical mode (Linux OSes have several different 'run levels', Where 1 is the most basic, 3 has everything except a GUI and 5 has everything including the GUI)
If you are new to Linux, you might want to try a couple of different distros to find one that suits. I personally quite like SuSE, since it is fairly beginner friendly, and hits a good balance of pre-installed apps (I seem to remember that Mandrake installed 3 or 4 different office suits and 3 or 4 PhotoShop clones and 2 web browsers by default)
Edit: When you have done all of the above you will now have discovered why Linux still isn't ready for the layman. Read the instructions for installing graphics card drivers on the ATI website for Linux and Windows, and you'll really see what I mean!
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