Dual Monitors under Windows 7

I recently upgraded my my Dell XPS M1530 to Windows 7 from Vista where I was running Dual monitors, the laptop screen and an external 20" monitor. After the upgrade I cannot seem to run two screens. If you left click on Screen resolution it only recognizes the external monitor but it stays black. It doesn't even see the laptop screen and still recognizes the external monitor even when it's not connected and turned off. The laptop monitor shows the desktop but again the laptop is only recognizing the external blank monitor

Any ideas

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