It used to be 'set in stone' that you would remove the previous Graphic card driver before installing a new one, and it's not hard to see why. I have always done that.
You can read the official line on this, at this page;
http://www.nvidia.com/object/IO_13955.html
But strangely on the same site, under FAQ, 'Do I need to uninstall my previous driver first?' the answer is 'no you don't'.
http://www.nvidia.com/object/drivers_faq.html
I had a 8400 card, and had several problems with the display settings, and only when I went back through some old magazine cover disks, to find a 8000 series driver, did it work perfectly.
So I would follow the instructions about how to remove older driver, then I would use the registry cleaner in 'CCleaner' to look for any entries left over.
Then, you should be able to use the
option 2 on the nvidia home page,
http://www.nvidia.com/Download/index.aspx?lang=en-us
to automatically find the correct version for your pc.
If you find it's no better, then repeat, but select from
option 1.
Hope that helps. If you get no better results after this,I could give the driver version I used, but try the above first.