Originally Posted by danton
I have the same problem on my m90. I found a solution that seems to work.
Disclaimer: I'm not really sure what the registry value modified below does, nor can I guarantee that changing it will not have any side effects
The solution is to modify LidBehavior registry value in nvidia card registry key from 0x11 to 0x10:
Edit: Most likely this will break dual display support in some way.
With the Dell Stock Drivers, e1705, 7900GS - CONFIRMED. This worked for me, just did it.