Then it must be an issue with the certificate repository of your OS, as 64 bit windows only allows signed drivers to run. Not using windows update, aren't we?...
You may want to install updates or temporarily disable signature enforcement and see if the driver runs then. Note that it IS signed, it's your system falsely assuming it is not.
CPU: AMD Phenom II X4 810 @ 3120MHz | RAM: Kingmax 2x2GB DDR2 800 @ 833MHz| MoBo: MSI K9A2 CF v1.0 (BIOS: 1.D)| GPU: Asus HD 6850 1024MB (DirectCu) @ 835/1135MHz | Display: L24FHD | PSU: PC Power & Cooling Silencer 750 Quad | OS: MS Windows 3.11 Pro x64