Update 11_20_09: Not everyone will need to reinstall the Guest OS, if you are confident the OS in your VM is supported (probably most of you) , skip to install integration services.
Also, because we have more than 1 releases version of Hyper-V now, it is possible for the integration services on the VM to be out of sync with the Hyper-V version. This article on TechNet discusses the various combinations and outlines what the support story is for each.
Some customers seem to be having trouble when they open up device manager in a VM and see that some devices are listed as “unknown device.” From the Troubleshooting Hyper-V topic on TechNet:
Cause: Device Manager does not recognize devices that are optimized for use in virtual machines and run using Hyper-V until integration services are installed. The unknown devices that are identified in Device Manager differ depending on the guest operating system and may include: VMBus, Microsoft VMBus HID Miniport, Microsoft VMBus Network Adapter, and storvsc miniport.
Some optimized devices are not available for certain guest operating systems. When a device is not supported on a guest operating system, the device will not work. You cannot install the device driver manually to try to make the device work. For a list of the devices that are available on each supported guest operating system, see http://go.microsoft.com/fwlink/?LinkID=128037.
Solution: If the guest operating system is supported, integration services are available for that operating system. After you install the integration services, Device Manager will recognize the devices that are available for that guest operating system.
To install the guest operating system
Open Hyper-V Manager. Click Start, point to Administrative Tools, and then click Hyper-V Manager.
Connect to the virtual machine. From the Virtual Machines section of the results pane, using one of the following methods:
The Virtual Machine Connection tool opens.
From the Action menu in the Virtual Machine Connection window, click Start.
The virtual machine starts, searches the startup devices, and loads the installation package.
Proceed through the installation.
Depending on the operating system being installed, the mouse pointer may change to a small dot when you move the mouse cursor over the image of the setup window. If this occurs, click anywhere in the virtual machine window. This action "captures" the mouse so that keyboard and mouse input is sent to the virtual machine. To return the input to the physical computer, press CTRL+ALT+LEFT ARROW and then move the mouse pointer outside of the virtual machine window.
Hyper-V includes a software package for supported guest operating systems that improves integration between the physical computer and the virtual machine. This package is referred to as integration services. Newer versions of supported Windows operating systems include the integration services and do not require installation after you install the guest operating system. For more information about which operating systems are supported and which of those require you to install integration services, see the deployment content for Hyper-V at the Windows Server 2008 Technical Library (http://go.microsoft.com/fwlink/?LinkID=128037).
The following instructions assume that you are installing an operating system for which the software package is available and installation is required.
To install integration services
From the Action menu of Virtual Machine Connection, click Insert Integration Services Setup Disk. This action loads the setup disk in the virtual DVD drive.
Depending on the operating system being installed, you may need to start the installation manually. Click anywhere in the guest operating system window and navigate to the CD drive. Use the method that is appropriate for the guest operating system to start the installation package from the CD drive.
After the installation finishes, all integration services are available for use.