Had the same problem with Windows 7 on the Cupertino2 mobo and GTX 760.
Fond a solution by accident. I dont know whether step 1 or 2 are essintial, but since I had performed them before they are included here:
1: Upgrade the HP bios from v7.14 to v7.16. I have done it with no GPU in the PCIe slot. Used the mobos VGA connector for the screen.
2: Delete all Nvidia files and drivers incl. registry.
3: Build in the GTX760. Make sure both the 8 and the 6 connectors cable from the PSU (600W) is connected to the graphic card.
4: Switch on the PC. As soon as the Bios screen is coming up press F12
5: New black screen with white letters and some information on configuration. At the lower right side it should show "Network Boot".
6: Wait 1 minute. There come 2 beeps with about 1/2 minutes interval.
7: Some 15 seconds after the second beep press "ESC" and keep on pressing. After some seconds some rapid cracking sound from the PC. Dont let it disturb you. Keep on pressing "ESC".
8: After the third beep the screen gets black and a white cursor blinking on the upper left. But: Keep on pressing "ESC" with no interruption.
9: After some seconds again a new screen appears with some information in DOS style. Here you can stop pressing "ESC".
10: Again black screen a few seconds, and then: Voila! Windows 7 is starting.
I dont know why its working, but it does for me. No new mobo needed