I used neosmart's tool to disable driver signature verification, but that didn't help.
So you're running Windows. Unless using older drivers or modified newer ones, Windows 7 is limited to 4 7900-series video cards and Windows 8 is limited to 5. I assume since you're disabling driver signature verification that you're already using modified drivers?
https://bitcointalk.org/index.php?topic=193695.0