If I'm remembering right, you should be able to run the file you downloaded-it should create a directory under like C:\Nvidia\<driver version> or something like that.
It'll then launch setup, which will fail, saying there's no supported hardware or something like that.
Instead, in device manager you can just tell it you want to update the video driver, and point the video driver to the inf file in the directory it created. It'll give a warning that there's no supported hardware or something like that, and you can just pick something similar to what the hardware actually is-an 8600GT maybe, or something along those lines (I don't know that that actually matters even).
When I did it, it took a few minutes copying files, then switched to a generic display driver an asked to reboot. It then came back up just fine, using the new drivers.
This was a year ago the last time I did this, but it seemed to work just fine and I'd assume it still does. I'm personally just more comfortable doing it that way versus using an unknown modded inf file (which all it's doing is telling the system that drivers do support the hardware...which doesn't matter since you can tell the system to use them anyway).
But you'd want to update the chipset drivers and DX 9 first, as the video drivers rely on the chipset drivers to function correctly.