Nvidia Windows Kernel Mode Driver Version 310.90 Download ##VERIFIED##
If I had to guess, I'd think this is a driver issue with NVidia - but, I can use 2.65a with any of the three NVidia drivers from 310.90 to 314.21 and Blender materials working perfectly while using Viewport Shading set to Rendered under Cycles. If this were purely a driver issue, then why does 2.65a work perfectly while all 2.66 versions (as far as I've tested) are not working - at least for me.
Nvidia Windows Kernel Mode Driver Version 310.90 Download
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