First thing you want to try is just removing the graphics card and remounting it.
If that doesn't help try disabling the driver signature enforcement.
1. Go to the power icon via the Start menu.
2. Hold on the Shift key and select Restart.
3. Click the Troubleshoot.
4. Click the Advanced options.
5. Click the Startup Settings.
6. Click the Restart button.
7. Press the F7 key to select the Disable the driver signature enforcement.
Then repeat steps 1-6 and select safe mode with networking.
Try installing the drivers while in safe mode.
Let us know how you get on.