i tried with switching to my CPU's GPU Intel Core i3-2100(HD Graphics 2000), removed the nvidia drivers and installed the intel driver from windows update and no more black screens(no on login, no on window drag over the taskbar).
when i read your comp conf. i realized that it maybe a nvidia driver bug. what driver version are you running. i am with latest nvidia beta drivers 320.00 on my NVIDIA Geforce GTX 460 SE
i have a problem with 0.91(RC0). when i drag a window and it starts to enter taskbar space i get a black screen. even on autohide taskbar when it is 1 pixel when window crosses it - instant black screen. with v0.9(beta3) i don't have this problem. sorry for bad English.