I am having this exact same problem after upgrading to the 15.23 chipset. It has been a few months since you posted so I am guessing you fixed it. I have been checking the Evga and Nvidia forums but couldn't find the answer.
I also have this exact same issue. I installed a second 8800 512GTS and then tried to upgrade to the 15.23 drivers, when it uninstalled the old drivers and rebooted it will not get to the log in screen. I then try and boot in safe mode and i can get to the log in screen. but the mouse and keyboard dont work. There is power going to both the mouse and keyboard, and the keyboard works in the BIOS just fine. Any help would be greatlly appreciated.
Is there any way that i can restore the old motherboard drivers?