Multiple monitors are not enabled on a computer that you upgrade from Windows XP to Windows Vista

When you upgrade a Microsoft Windows XP-based computer to Windows Vista, multiple monitors are not enabled. This issue may occur if the computer has more than one graphics adapter installed.

This issue may occur because the Windows Vista Setup program detects and configures only one graphics adapter. You expect all the installed graphics adapters to be configured in Windows Vista.

To support multiple graphics adapters in Windows Vista, the following conditions must be true:

• All the graphics adapters must use the same driver model. Therefore, all the graphics adapters must use Windows XP Display Driver Model (XDDM) drivers. Or, all the graphics adapters must use Longhorn Display Driver Model (LDDM) drivers.
• If the graphics adapters use Windows Vista Display Driver Model (WDDM) drivers, all the graphics adapters must use the same driver. For example, you may have two graphics adapters that use the same driver. Or, you may have a Dualview graphics adapter. For more information about Dualview, visit the following Microsoft Web site:
  http://www.microsoft.com/windowsxp/evaluation/features/dualview.mspx

To work around this issue, use a multiple graphics adapter configuration that supports WDDM drivers. To support multiple graphics adapters in Windows Vista together with WDDM drivers, use one of the following configurations:

• Use a Dualview graphics adapter.
• Use a dual-lane PCI Express computer system that has two homogeneous graphics adapters.

Note For more information about these configurations, see the "More Information" section.

Multimon (an abbreviation for Multimonitor) functionality may be described as follows:

• Dualview
  
  This term describes a computer that has a single graphics adapter that has two video outputs. This kind of adapter is also known as a dual-head graphics adapter. Many graphics adapters have a DVI output together with a VGA output.
• Homogeneous multi-adapter
  
  This term describes a computer that has two graphics adapters that use the same driver. For example, the computer has an NVIDIA GeForce FX 5600 graphics adapter and an NVIDIA GeForce FX 5950 graphics adapter.
• Heterogeneous multi-adapter
  
  This term describes a computer that has two graphics adapters that use different graphics drivers. For example, the computer has an ATI Technologies Radeon 9800 series graphics adapter and an NVIDIA GeForce FX 5600 graphics adapter.

The third-party products that this article discusses are manufactured by companies that are independent of Microsoft. Microsoft makes no warranty, implied or otherwise, about the performance or reliability of these products.