You appear to be doing everything right. Of course, that only proves that we're both missing something.
I couple of thoughts:
1. Is there a video card built onto the motherboard that could be conflicting? (An oddball question, but one that I should ask.)
2. Most likely, those aren't
exactly the right drivers. What
exactly does the screen tell you when you boot the PC?
Here is the download site for
nVidia Drivers.
What version of Windows are you running?