I know this thread is old.
But I ahve this exact same problem with my Vista machine.
I have a GeForce 8500 gt with a vga port and a dvi port.
I have a monitor (Main -monitor 1) which I connect using vga
And a second monitor which I want to connect using dvi.
However I get "no signal" from my dvi monitor.
If I try to use my dvi monitor by itself as a single main monitor, I can see the bios screen, and the windows load bar, but when it gets to the windows login screen I get "no signal" again, and the screen reverts back to d-sub mode as opposed to DVi that it was previously on in the bios screen.
I've tried installing several different versions of the nvidia drivers. -all clean installs.
I cannot find a solution anywhere :/