After re-reading your original post, you have done all the proper methods.
I believe there may be a leftover driver from the original Windows update that was first installed.
Go to control panel > Programs and features, click the view installed updates and see if it's listed here.
If not, create a restore point.
open Regedit and do a search for nvidia. Delete all references to nvidia if found.
Do a search of the hard drive for nv*. If you locate a file named nvlddmkm.sys, delete that file as well.
Restart your computer and then try to install the Windows 7 driver from Nvidia.
I highly doubt that Windows will not start, but if it doesn't, hit the F8 key (may be different depending on manufacturer) until you see the...